Higher intakes of calcium and dairy products, a major source of dietary calcium, are reported to increase the risk of prostate cancer, potentially due to reductions in circulating vitamin D with increasing calcium intake. We prospectively examined the association of dairy product and calcium intake with prostate cancer risk in 29,509 men, including 1,910 cases, in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. We also evaluated the relation of calcium intake with serum 25-hydroxy-vitamin D [25(OH)D] and 1,25-dihydroxy-vitamin D [1,25(OH)(2)D], in a Prostate, Lung, Colorectal, and Ovarian Trial substudy (n = 275). Dietary intake was assessed using a food frequency questionnaire. Baseline serum 1,25(OH)(2)D was determined by RIA. Greater intake of dairy products, particularly low-fat dairy products, was weakly associated with increased risk of prostate cancer [relative risk (RR), 1.12; 95% confidence intervals (CI), 0.97-1.30; P trend = 0.06 for >2.75 versus < or = 0.98 servings of total dairy/day; 1.23 (1.07-1.41) for low-fat dairy]. Greater dietary calcium intake was associated with increased risk of prostate cancer (RR, 1.34; 95% CI, 0.93-1.94; P trend = 0.02 for >2,000 versus <1,000 mg/day), but greater supplementary calcium intake was not associated with the risk. Associations of dairy product and dietary calcium intake were evident for nonaggressive disease (RR, 1.20; 95% CI, 0.99-1.46; P trend = 0.01 for dairy products; 1.64, 1.04-2.57; P trend = 0.002 for dietary calcium), but not aggressive disease (RR, 1.02; 95% CI, 0.81-1.28 for dairy products; 0.94, 0.49-1.80 for dietary calcium). Calcium intake was not associated with serum 25-hydroxy-vitamin D and 1,25(OH)(2)D concentration. In this large prospective study in a prostate cancer screening trial, greater dietary intake of calcium and dairy products, particularly low-fat types, may be modestly associated with increased risks for nonaggressive prostate cancer, but was unrelated to aggressive disease. Furthermore, we found no relationship between calcium intake and circulating vitamin D.