A day after the S&P 500's first close above 2,000, veteran trader Art Cashin told CNBC on Wednesday the rally could soon lose steam, at least if history is any indicator.
"Unfortunately, history gives us a kind of muted picture of it, that if you have high volume, you get follow-through very easily. If you have low volume and you go into September, we've only had big carry-through a few times," Cashin, director of floor operations at the NYSE for UBS, said on "Squawk on the Street."
Much of the recent gains have come on low volume, suggesting investors are either wary of jumping in at current levels, or have simply gone on summer vacation.
The gains have come without any correction since 2012, even amid such recent potential headwinds as the conflict between Ukraine and Russia and the possibility that European Central Bank President Mario Draghi might direct the ECB to embark on a large-scale asset-buying scheme to revive inflation.
"Now we're in unusual times these days and we'll wait to see if Mr. Draghi actually delivers. We're at the 'where's the beef?' stage and see what's going to happen," Cashin said. "I think you can have this going on. The yields in Europe are so low that you're going to have some European buying of U.S. bonds."
For a read on the markets, Cashin recommends investors watch the yield on the 10-year Treasury note, which fell 3 basis points to 2.366 percent in midday trading Wednesday.
—By CNBC's Drew Sandholm. Reuters contributed to this report.