WASHINGTON — The long decline in sexual activity among American teenagers, hailed as one of the nation’s most important social and public health successes, appears to have stalled.

After decreasing steadily and significantly for more than a decade, the percentage of teenagers having intercourse began to plateau in 2001 and has failed to budge since then, despite the intensified focus in recent years on encouraging sexual abstinence, according to a new analysis of data from a large federal survey.

The halt in the downward trend coincided with an increase in federal spending on programs focused exclusively on encouraging sexual abstinence until marriage, several experts noted. Congress is currently debating funding for such efforts, which receive about $175 million a year in federal money and have come under fire from some quarters for being ineffective.

The leveling off in teenage sexual activity is worrying experts and advocates across the ideological spectrum. The fall of such activity has been one of the key forces behind a historic drop in teenage pregnancy rates and has bolstered efforts to protect teenagers from sexually transmitted diseases.

“It is alarming,” Susan Philliber of Philliber Research Associates, a private firm that studies teenage sexuality, said. “We’ve had such a wonderful decade’s run of getting the rate down. For it to level out causes everyone to go, ‘Uh-oh.'”

Experts are unsure of the reasons for the change, but they speculated that it could be due to a combination of factors, including growing complacency among the young and the possibility that some irreducible portion of the teenage population can never be dissuaded from having sex.

“At a certain point, it becomes really hard to change basic human behaviors,” John Santelli, who studies teenagers at Columbia University, said. “I think what we’re seeing is the limits of the emphasis on abstinence as the primary message.”

But abstinence proponents argue that, if anything, the data underscore the need for greater emphasis on encouraging youngsters to abstain from sex until marriage. “We need to increase abstinence education and give more dollars to abstinence education. It is the healthiest program we have for young people,” Leslee Unruh of the National Abstinence Clearinghouse said.

A recent study of four separate abstinence programs, conducted for the Department of Health and Human Services by Mathematica Policy Research, a nonpartisan firm, found no evidence that the programs delayed the start of sexual activity among teenagers, but Ms. Unruh and others said such programs need more time and wider use to counter pervasive messages encouraging teenagers to have sex.

“Teenagers today live in an MTV-driven culture and are bombarded by sexual messages that say it is normative for them to get involved sexually,” Charmaine Yoest of the Family Research Council said. “We need a message that sexual experimentation as a teenager is unhealthy.”

The House last week approved a $28 million increase in spending on abstinence programs — Democratic leaders said it was intended to win Republican support for the annual health and education funding bill — but the Senate is considering a $28 million cut, largely because of concerns about the programs’ efficacy.

The proportion of teenagers reporting having sexual intercourse rose steadily throughout the 1970s and 1980s, fueling an alarming rise in teenage pregnancy. But that trend reversed around 1991 because of rising fears about sexually transmitted diseases; changing social mores about sexual behavior, and other factors. At the same time, more teenagers reported using condoms and other forms of contraception. Together, the trends have pushed the teenage pregnancy rate to historic lows, especially among African Americans.

“If there has been more progress made on another difficult major social issue, I don’t know what it is,” Bill Albert of the National Campaign to Prevent Teenage Pregnancy said. “It’s really a remarkable achievement.”

But because the American teenage pregnancy rate remains far higher than those in other developed countries — double the rate in England, for example — the leveling off of sexual behavior is triggering alarms.

“Although we can celebrate the decline in teenage pregnancy, the United States still has a very high level of teenage births and pregnancy compared to other industrialized nations,” David Landry of the Guttmacher Institute in New York, an independent research institute that studies reproductive health issues, said. “There’s still a long way to go.”

Some fear that the leveling off in the rate of teenagers having intercourse could foreshadow a new upsurge in sexual activity, which could cause teenage pregnancy and birth rates to climb again.

“My concern is that this plateau is the canary in the coal mine — a harbinger of a reversal of these positive trends,” Michael Resnick, a teenage-health expert at the University of Minnesota, said.

The survey indicates that the increase in condom use may also be waning. “It’s not showing as strong of a stabilizing trend, but it’s definitely slowing,” Mr. Landry said. The data on teenage sexual activity comes from the Youth Risk Behavior Survey, a nationally representative survey of about 13,000 students in grades nine through 12 conducted every two years by the federal Centers for Disease Control and Prevention.

The most recent survey data, from 2005, was released last year, but attention focused primarily on the overall change in sexual behavior from 1991 to 2005 — both when the CDC initially reported the data in June and again earlier this month when the findings were highlighted in an annual federal report on the well-being of American children. That comparison shows a significant drop, from 54% to nearly 47%, in the proportion of teenagers who said they had ever had sex. The fraction who said they had had sex in the past three months fell from 37% to 34%.

Largely unnoticed was that the percentages for both measures did not change significantly between 2001 and 2005. In response to a request from the Washington Post, the CDC analyzed the data for that time period to validate statistically that the rates had leveled off, for all grades, for both boys and girls, and across all racial groups. “We found that over that time period, there were no changes in the overall percent of high school students who had ever had sex,” Laura Kann, who heads the survey project at the CDC, said. “It has decreased over the whole time from 1991, but there’s been no change since 2001. There was flatness for all the subgroups for all the variables.”

Researchers are awaiting the data from this year’s survey to see whether the trend has continued. Another analysis of the available data comparing the results from two time periods — between 1991 and 1997 and between 1999 and 2005 — supports the interpretation that the trend line flattened. “It seems clear that the trend seems to be faltering in the most recent period of time,” said Columbia’s Mr. Santelli, who plans to present his findings this fall at a meeting of the American Public Health Association.