Read MoreJeremy Siegel: Here's what will send yields higher
"I think we're going to get to 18 and above. Could it go to 19, 20? It could," he said. "I'm not going to say that's the likely event, but so many people have missed this bull market that they start saying, 'Hey, you know, this is my last chance.'"
But Siegel saw a limit for stock prices.
"We could be overvalued at 20-21,000, at least in the short run," he added. "Again, just like I said a year ago, markets often go beyond fair market value before they correct back down."
Read MoreThis could derail my Dow-at-18,000 call: Jeremy Siegel
Siegel also said that there "could be a lot of volatility" in the next six to 12 months.
A few factors could still derail the rally, he added.
"I would say my biggest worry would be on the inflation front. If we start seeing some supply constraints, if the low unemployment brings wage increases that are not matched by productivity increases, oil stays high and gets higher, that'll be a problem," he said. Much higher gasoline prices, too, "would certainly be a problem."
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—By CNBC's Bruno J. Navarro