WASHINGTON — Doctors discovered a blood clot in Vice President Cheney’s left leg yesterday, a condition that could be fatal if left untreated.

The 66-year-old Mr. Cheney, who has a history of heart problems, will be treated with blood-thinning medication for several months, said spokeswoman Lea Anne McBride.

She said Mr. Cheney visited his doctor’s office in Washington after feeling minor discomfort in his calf. An ultrasound showed the blood clot — called a deep venous thrombosis — in his left lower leg.

Blood clots that form deep in the legs can become killers if they break off and float into the lungs. This is called a pulmonary embolism. Deep vein thrombosis strikes an estimated 2 million Americans each year, killing 60,000.

Many people suffer DVT after spending long periods without moving, such as passengers on long-haul airline flights. Mr. Cheney spent about 65 hours on a plane on a nine-day, round-the-world trip that ended last week.

“When you’re just sitting there with your legs hanging down for long periods of time, that’s what predisposes you to the problem,” said Sean O’Donnell, chief of vascular surgery of Washington Hospital Center.

“Treated properly, it poses a small threat,” Dr. O’Donnell said. “Untreated and unrecognized, it’s a very serious problem.”

NBC correspondent David Bloom died in 2003 of a pulmonary embolism after spending days in a cramped military vehicle while covering the invasion of Iraq.

Airlines often encourage passengers to walk the aisles and wiggle their feet. But flying’s not the only risk of DVT; it extends to anyone older than 60 or who has heart failure. Recent surgery, fractures, childbirth, and birth control pills also can raise the risk.