The associations between recreational and occupational physical activity and the subsequent risk of prostate and testicular cancer were examined in a population-based cohort study of 53,242 men in Norway. Age at study entry was 19 to 50 years. Information on physical activity was based on questionnaire responses and a brief clinical examination. A total of 220 prostate and 47 testicular cancer cases were recorded in the Cancer Registry of Norway during a mean follow-up time of 16.3 years. We found a nonsignificant, reduced, adjusted relative risk (RR) of prostate cancer with increased level of physical activity at work and among those men with the greatest recreational physical activity. When occupational and recreational physical activity were combined, a reduced adjusted risk of prostate cancer was observed among men who walked during occupational hours and performed either moderate recreational activity (RR = 0.61, 95 percent confidence interval [CI] = 0.36 to 1.01) or regular recreational training (RR = 0.45, CI = 0.20 to 1.01) relative to sedentary men (test for trend, P = 0.03). Physically active men who were older than 60 years of age at diagnosis showed a reduced adjusted RR of borderline significance, while no association was observed for younger men. No evidence was found for any association between physical activity and testicular cancer regardless of physical activity at work and recreation.