Longtime stock bull Jeremy Siegel warned on CNBC on Monday that the markets may be in for a correction by summer if the Fed moves to raise interest rates.
"It's the first time I've been cautious in a while," he said in a "Squawk Box" interview on the sixth anniversary of the market bottom.
Siegel also said he considers 20,000 on the Dow Jones industrial average as fair market value. But with blue chips about 10 percent away, he said he doesn't see a straight shot to Dow 20,000.
"It may not get there in 10 months by December," Siegel said.
"If we're going to get a June rise, there are going to be some ripples in the stock market before this is over," he said. "I certainly would not be surprised to see a correction in the next three months that brings the market, down maybe 5 percent to 10 percent."
Following the financial crisis, the S&P 500 index set an intraday low of 666 on March 9, 2009. Since then, the index has risen 206 percent, the Dow Jones industrial average has climbed 173 percent, while the Nasdaq composite has gained 288 percent.
"On a historical basis, the current price [to] earnings ratio is above that historical average. The reason I'm not bearish is that interest rates are so low," Siegel said. "Even with the Fed maybe tightening in June, maybe September, they are going to remain well below their average for many, many years." He added that stocks must be valued relative to rates and the investment alternatives and not on PE ratios alone.
The Wharton professor said he still likes stocks at these levels because with just modest earnings and dividend growth the market should still return around 6 percent compared with zero to 2 percent in bonds. "I'm still bullish, long run."
Last Monday, March started off with the Nasdaq rallying above 5,000 for the first time in 15 years,and the Dow industrial average and S&P closing at record highs. Optimism was in short supply as the week progressed, however, with all three measures sharply lower Friday after the much stronger-than-expected February jobs report sparked concern about an earlier Fed rate hike.