This study evaluates the risk of prostate cancer in relation to serum levels of the major vitamin D metabolites, 25-hydroxyvitamin D (25-D3) and 1,25-dihydroxyvitamin D (1,25-D). Between 1964 and 1971, more than 250,000 serum samples were collected from members of the Kaiser Permanente Medical Care Plan in Oakland and San Francisco and stored for future use. Levels of 25-D and 1,25-D were measured in samples from 90 black and 91 white men diagnosed with prostate cancer before December 31, 1987 and controls individually matched on age, race, and day of serum storage. Mean serum 1,25-D was 1.81 pg/ml lower in cases than in matched controls (P = 0.002). Risk of prostate cancer decreased with higher levels of 1,25-D especially in men with low levels of 25-D. However, mean 25-D was not significantly different in cases and controls. The association of lower 1,25-D with prostate cancer was found in men above the median age of 57 years at serum storage but not younger men and was similar in black and white men. In men > or = 57 years of age, 1,25-D was an important predictor of risk for palpable and anaplastic tumors but not for tumors incidentally discovered during surgery to treat the symptoms of benign prostatic hyperplasia or well differentiated tumors.