We previously found that statin users had a lower risk of advanced and possibly high-grade prostate cancer compared with nonusers. We hypothesize that statins' effects on cholesterol synthesis may explain those findings because prostate cancer cells exhibit cholesterol dysregulation. Thus, we investigated whether low plasma cholesterol is associated with prostate cancer overall and by stage and grade. Participants were drawn from the 18,018 members of the Health Professionals Follow-Up Study who provided blood in 1993-1995. We ascertained 698 incident cases through January 2000. Controls were 698 men who had a PSA test and were matched to cases. Plasma cholesterol was measured enzymatically. Conditional logistic regression was used to estimate multivariable ORs and 95% CIs of total, clinically organ-confined (n = 518), advanced (T3b or worse; n = 61), low-grade (Gleason sum < 7; n = 386) and high-grade (Gleason sum > or = 7, n = 247) disease. Low cholesterol (<25th percentile vs. > or =25th percentile) was not associated with total (OR = 0.93, 95% CI: 0.72-1.20), organ-confined (OR = 0.87, 95% CI: 0.64-1.18) or low-grade (OR = 1.06, 95% CI: 0.75-1.51) disease. However, men with low cholesterol had a lower risk of high-grade disease (OR = 0.61, 95% CI: 0.39-0.98), especially if organ-confined (OR = 0.54, 95% CI: 0.29-0.99). The association for advanced disease appeared inverse, but number of cases was small (OR = 0.42, 95% CI: 0.13-1.36). Associations remained after excluding cholesterol-lowering drug users. These results coupled with prior statin findings suggest that mechanistic studies on cholesterol metabolism should be pursued to understand a possible target for preventing poorly differentiated prostate cancers.