It’s one of those dreadful anniversaries: the bloodiest one-day showing in the history of the American stock market, when the Dow Industrial dropped a wicked $500 billion, or 22.6%. I am talking about October 19, 1987, or Black Monday, which will mark its 20th anniversary in just a few weeks.

Given the current market’s bevy of troublesome economic and financial questions, the possibility of another Black Monday is being discussed throughout a predominantly bullish but jittery Wall Street. That’s a legitimate worry, some say, in light of the undeniable risks. Chief among them:

• The sudden inability to roll over some of that $1.9 trillion of commercial paper, about half of which is in the more vulnerable asset-backed paper market (such as auto and housing loans).

• Much faster credit tightening, with a real danger, some say, of overtightening.

• Increasing difficulty of small businesses to raise capital.

• Accelerating weakness in the battered housing market, characterized by a surging number of foreclosures, defaults, delinquencies, and unsold homes on the market. Likewise, prices are falling, housing starts are at a 12-year low, and warnings are widespread that adjustable rate mortgage resets to higher rates could wreak new havoc on the market.

• Widespread uncertainty about just how much of the $1.3 trillion of subprime mortgage debt has turned sour.

• Growing fears of a sharp slowdown in consumer spending, which could precipitate rising unemployment and a recession.

• The sizable deterioration in America’s relations with the rest of the world.

“The clear message is there’s a lot to fret about,” investment adviser Bill Rhodes of Boston-based Rhodes Analytics says. “Last week’s recent rate cut,” he adds, “has reduced the likelihood of a commercial paper problem, but it hasn’t eliminated it.”

A Los Angeles money manager, Arnold Silver of A. Silver Associates, says he views the Federal Reserve’s rate reduction as a step in the right direction, but he says he doubts it will eliminate growing credit and mortgage woes in any speedy fashion. It’s way too early, he says, to conclude the damage in housing and mortgages won’t spill over into the rest of the economy. He also questions the market’s buoyant response to the rate cut, observing, “Investors are acting like the cancer has been totally eliminated, which is not so.” Other worries are the possibility of rising inflation and a recession.

Interestingly, even the bulls, such as Standard & Poor’s chief investment strategist, Sam Stovall, are not suggesting the market is home free. “Future shocks could upend the market near term,” he says. “It’s not out of the woods; there are lurking dangers.”

Among them, he cites the possibility of disappointing third- or fourth-quarter earnings stemming from major write-downs, including the subprime area. Likewise, he points to prospects of a consumer spending slowdown because of the rate resets in adjustable rate mortgages.

Still, Mr. Stovall says, the market positives outweigh the negatives. He notes in particular the Fed’s rate cutting, the likelihood that the economy will not swing into a recession, the stimulus of global economic growth, and his expectation of accelerating, not slowing, earnings growth in 2008. As such, he expects the S&P 500 to wrap up 2007 at 1,550, a 9% gain for the year. The index closed Friday at 1,525.75.

Meanwhile, let’s say he is right, and stocks could get upended in the short term and run afoul of one of those periodic market shocks. What’s an investor to do? Hold fast or, if you’re the gutsy type, buy, he says. How does he figure that? He’s checked the results of a study he conducted of six major shocks, including the September 11, 2001, attacks, the Kennedy assassination in November 1963, Iraq’s invasion of Kuwait in August 1990, and Pearl Harbor in December 1941. The average first-day decline of these market shocks ran 2.9%, but the number of days the market was in free fall was only six. Significantly, the S&P 500 traded at a new high just 55 days after the shock, or only 15 days if you omit Pearl Harbor.

Any brave soul who bought on Black Monday, 1987, based on the subsequent recovery racked up a nifty 16.5% increase in the ensuing six months and an even fatter 20.4% gain a year out. In other words, not panicking paid off big time.

The lesson to learn, Mr. Stovall says, is buying in the face of shocks is a good way to get rich slow.

Meanwhile, some of the similarities between the 1987 crash and today’s environment are striking. Among them, in October of that year there was a huge trade deficit and a falling dollar; likewise, heavy use of derivatives. In contrast, interest rates were rising then, while they are falling now. Also, prior to the crash, the S&P 500 traded at 19.6 times trailing 12-month earnings; the P/E is now 17.7.

What do I think? I go along with Mr. Rhodes: There’s a lot to fret about.