The increase in the incidence rate of prostate cancer may be associated with changes in lifestyle in Japanesemen. Accordingly, we conducted a case-control study to assess risk factors. A total of 117 (82.3%) of the 142prostate cancer patients asked filled out the self-administrated questionnaires which included items abouttheir lifestyle habits over the period of one or two years before their diagnosis. Four controls per case, namely468, were randomly selected from resident registries with age and address matched with each case, and 318controls (69.5%) filled out the same questionnaire as the cases. Data for 277 controls were used for the analysis,excluding 41 subjects with a history of previous cancer. The conditional logistic regression model was utilizedfor analyzing the individually age and address-matched data, and odds ratios (ORs) and their 95% confidenceintervals (95%CIs) were calculated for potential risk factors. Higher body mass index at 20 years of age wasmarginally significantly associated with a decreased risk (P for trend=0.051), and larger weight gain in adultage was significantly associated with an increased risk (P for trend=0.041). History of prostate cancer in fathersor brothers was significantly associated with an increased risk (OR=9.71, 95%CI 3.59, 26.27), and history ofbreast cancer in mothers or sisters was also significantly associated with an increased risk (OR=2.70, 95%CI1.12, 6.49). The recent increase in the incidence rate of prostate cancer may possibly be brought about by anincreased proportion of Japanese men with large weight gain in adult age.