Stocks Haven’t Been This Expensive Since 1999. The Last Time It Happened, They Fell 50%.

The U.S. stock market is approaching a valuation milestone not seen since the dot-com era, reviving questions about whether investors should brace for a repeat of one of Wall Street's most painful downturns.

The S&P 500's Shiller price-to-earnings ratio, also known as the cyclically adjusted P/E or CAPE ratio, recently reached 42.2, according to a report from The Motley Fool. That is the highest level in more than 26 years and puts the closely watched valuation measure within striking distance of its November 1999 record of 44.2.

The comparison is notable because the last time valuations climbed this high, the technology-driven dot-com boom was approaching its peak. But today's market is different in important ways, particularly because many of the technology companies commanding premium valuations are highly profitable businesses rather than speculative startups.

The CAPE ratio measures the price of the S&P 500 relative to the inflation-adjusted earnings of its companies over the previous 10 years. By using a decade of earnings, the measure attempts to smooth out temporary economic disruptions that can distort conventional price-to-earnings ratios.

A higher CAPE generally indicates investors are paying more for every dollar of corporate earnings. Since the beginning of 1990, the ratio has averaged just above 27, according to The Motley Fool. A reading above 42 therefore places current valuations far outside the market's recent historical norm.

That does not mean a crash is imminent. The historical parallel nevertheless carries a warning. The S&P 500 peaked at 1,527 in March 2000 as enthusiasm for internet companies reached extraordinary levels. Over roughly the next two and a half years, the index lost about half its value as the dot-com bubble burst, wiping out companies and billions of dollars in investor wealth.

Enthusiasm surrounding artificial intelligence has helped propel valuations for some of America's largest technology companies and increased the S&P 500's dependence on a relatively small group of mega-cap stocks.

The so-called "Magnificent Seven" have become particularly influential because of their enormous market capitalizations and investors' willingness to pay premiums for their expected growth.

That concentration means movements in a handful of companies can have an unusually large impact on the broader index. There is also a critical difference between the current AI boom and the internet frenzy of the late 1990s. Many dot-com companies attracted enormous valuations despite having limited revenue, uncertain business models and, in many cases, no profits.

The technology giants leading today's market are established companies generating substantial revenue and earnings. That makes a direct comparison with 2000 difficult, even if the CAPE ratio suggests investors are once again paying historically high prices for stocks.

While valuation measures can indicate whether stocks look expensive relative to history, they cannot reliably predict when a correction will occur. An expensive market can remain expensive for months or years, particularly if corporate earnings continue growing fast enough to justify higher stock prices. Conversely, valuations can fall through declining share prices, rising earnings, or a combination of both.