He feels they have farther to go:

Famed investor Jeremy Grantham said that he expects the S&P 500 (SP500) and the Nasdaq (COMP.IND) to fall about double from recent levels. “The other day we were down 19.9 on the S&P and about 27 on the Nasdaq,” Grantham said earlier in a CNBC interview. “I would say that at minimum we are likely to do twice that and if we are unlucky, which is quite possible, we will do three legs like that and it may take a couple of years, as it did 2000.”

Either way, things get worse before they get better. The only question is how much worse.