He called it a "stealth bear market" because it's something investors don't necessarily see because they're watching the big averages.
"Someone has his hand in your back pocket, picking your pocket from behind," he said Monday.
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Stocks ended higher Monday, with the small-cap Russell 2000 index leading the way, but Acampora said one good day does not change his thesis.
"I hope it's not a one-day wonder," he said. "I need to see across the board new highs."
However, this doesn't mean he's turned bearish. Instead he's looking for a correction in a secular bull market, and expects large-cap stocks to "settle down" and drop about 10 to 15 percent.
"If this happens between now and October, you wash it out of the way and then we're going to have a strong fourth quarter," he said.
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—By CNBC's Michelle Fox. CNBC's Alex Rosenberg contributed to this report.