An hypothesis has been forwarded linking prostate cancer to low serum levels of vitamin D metabolites. We sought to test this hypothesis using sera obtained in a large, prospective cohort study. A serum bank in Washington County, Maryland (United States) has stored sera obtained from 20,305 county residents during a blood collection campaign undertaken in August through November 1974. We studied sera obtained from 61 residents who were diagnosed with prostate cancer during the period 1980 through 1992. Each prostate cancer case was matched to two controls on age (+/- 1 yr) and race. Controls had donated blood in the same blood-collection campaign and had not been diagnosed with prostate cancer through 1992. Serum levels of vitamin D metabolites did not differ significantly between cases and controls. Mean 25-hydroxyvitamin D (25-D) levels were 34.3 ng/ml and 33.2 ng/ml, and mean 1,25-dihydroxyvitamin D (1,25-D) levels were 41.0 pg/ml and 40.1 pg/ml, in cases and controls, respectively. No statistically significant trends or differences between cases and controls were found in an analysis by quintile of serum level. We also did not observe the association of vitamin D metabolites with prostate cancer to be strongest among older men with more severe disease, as previously has been reported. In summary, although our study's power was limited, our findings provide little support for the hypothesis that vitamin D metabolite levels are associated strongly with subsequent risk for prostate cancer.