Using data from two case-control studies undertaken in Athens, Greece from 1994 to 1997, we have examined the association of occupational physical activity with the risk of prostate cancer and benign prostatic hyperplasia (BPH). Cases consisted of 320 patients with histologically confirmed incident prostate cancer and 184 patients with surgically treated BPH. Controls were 246 patients hospitalized for minor conditions. Occupations before retirement were classified, independently and blindly as to case-control status, into high, medium, and low physical activity levels. After fine controlling for years of schooling, there was a suggestive inverse association of physical activity with prostate cancer (P for trend 0.12) and a significant one with BPH (P for trend 0.04). The odds ratio (95% confidence interval) for high versus low activity was 0.69 (0.40-1.22) for prostate cancer and 0.59 (0.31-1.11) for BPH. The association of physical activity with both conditions tended to be more pronounced among men 65 years old or younger. Given the high frequency of occurrence of the examined conditions in the male population and our limited knowledge about other modifiable risk factors, preventive measures may have to focus on increasing physical activity.