But most experts say the wild swings are simply a sign of the boom times and not a harbinger of yearlong gloom.
For the first time in eight years, all of the broad stock market indexes fell in January, an ominous sign for those market watchers who use the so-called "January indicator" to predict the future.
Market lore has it that when the major indexes rise in January, stocks will gain over the full year, and vice versa.
Indeed, since 1950, the January indicator has only been wrong three times, according to Yale Hirsch, author of the Stock Trader's Almanac.
But the stock market is rife with such indicators, and most professional stock-pickers take them with a grain of salt.
"It's too early and there's too much mixed data out there to throw in the towel for the year," said Brian Belski, chief investment strategist at George K. Baum, a Kansas City, Mo., brokerage firm.
The Dow Jones Industrial Average, the most closely watched index, ended January at 10,940.53, down almost 5 percent from its close on Dec. 31, 1999. The average of 30 blue-chip stocks also was off almost 7 percent from the record 11,722.98 reached Jan. 14.
It also was the first January retreat since 1992 for the Standard & Poor's 500-stock list.
The broad S&P 500 index, favored by most Wall Street professionals, fell about 5 percent in the first month of the year, and the technology-heavy Nasdaq Composite Index slipped slightly more than 3 percent.
A number of factors contributed to those declines, not the least of which was investors simply taking profits after record-setting stock market gains during the fourth quarter of 1999.
In addition, the roaring U.S. economy has put Federal Reserve Board policymakers on guard for signs of creeping inflation, a threat that led to three interest rate hikes last year.
A fourth rate hike is widely expected Wednesday, and investors traded cautiously throughout January bearing that in mind.
Higher interest rates hurt stocks because they make it more costly for companies to borrow money to expand their businesses, cutting into earnings.
Notwithstanding the track record of the January indicator, few worry that one down month will derail the longest running bull market in history.
Belski said the wild mood swings exhibited by the stock market in recent months are typical of a full-blown bull market. Investors are edgy, he said, and prone to making quick decisions based on emotions. So the January selloff was hardly surprising, he added.
When taken in their proper context, some indicators can be helpful, analysts believe. But none should be considered some sort of crystal ball that can be used to shine a light into the future.
The presidential election indicator, for example, is based on a historical precedent that presidents tend to be more conservative in their last two years of office so as not to disturb the economy. Consequently, the status quo remains intact and stock markets go up.
During the 20th century, the Dow Jones average fell just seven times during 25 presidential election years, according to Hirsch's Stock Traders Almanac.
Hirsch said these types of gauges can be thrown off by extraordinary circumstances. "There's no such thing as a perfect indicator," he said.
The current interest rate environment, for example, held stocks down in January but probably won't affect the rest of the year, he said.
Stocks will likely climb higher in 2000 regardless of the January indicator because the U.S. economy is booming, company earnings are strong, overseas markets are healthy, and the Internet is changing how the world does business, Hirsch said.
One indicator that has no one worried is the so-called Super Bowl indicator, which suggests _ somewhat arcanely _ that when an original member of the old American Football League wins the Super Bowl in January the stock markets will tumble that year.
The Denver Broncos were original members of the AFL, however, and the team won consecutive Super Bowls in 1998 and 1999 in the midst of the current bull market.
Conversely, when the Super Bowl is won by an original member of the National Football League, which merged with the upstart AFL in the early 1960s, the markets are supposed to go up. But that indicator clashes this year with the pall cast by the January indicator since the St. Louis Rams, winner of Sunday's Super Bowl, were an original NFL member.
Robert Robbins, chief investment strategist at Robinson-Humprey, an Atlanta-based brokerage firm, dismissed football indicators outright.
"There's no reason to believe that football has anything to do with the stock markets," he said. "That's just something that's fun to talk about."