The present study explored body mass index (BMI), height, and risk of prostate cancer in a large Norwegian cohort of 950000 men aged 20-74 years, whose height and weight were measured in a standardised way in the period 1963-1999. These were followed for an average of 21 years. The Cox proportional hazard models were used in the analyses. During follow-up, 33 300 histologically verified cases of prostate cancer were registered. The risk of prostate cancer increased by both BMI and height. The magnitude of the increase by BMI was modest, the relative risk (RR) of obese men (BMI>or=30) compared with normal weighted was 1.09 (95% CI: 1.04-1.15). However, the RR at age 50-59 years was 1.58 (95% CI: 1.29-1.94) in men being obese at about age 45 years compared with normal weighted men. The tallest men had an RR of 1.72 (95% CI: 1.46-2.04) compared with the shortest men. The overall effect of BMI on the incidence of prostate cancer was modest. The larger effect found in men aged 50-59 years might partly explain the previous inconsistent findings.