NEW YORK — Investors piled into stocks on the first trading day of 2010, pushing the Dow Jones industrial average up 1.5 percent to its highest level in 15 months — raising hopes that stocks can continue the rally that began in March after the worst swoon since the 1930s.

The market's performance in the first five sessions of the new year will be closely watched on Wall Street, as it is viewed as an "early warning system" as to how the market will fare the rest of the year. The last 37 times the broad U.S. stock market posted gains in the first five days of trading, it was followed by full-year gains 32 times, a success rate of almost 90 percent, says the Stock Trader's Almanac.