The age-adjusted incidence of prostate cancer is low in Japan, and it has been suggested that the traditional Japanese diet, which includes many soy products, plays a preventive role against prostate cancer. We performed a case-control study on dietary factors and prostate cancer in order to assess the hypothesis that the traditional Japanese diet reduces the risk of prostate cancer. Four geographical areas (Ibaraki, Fukuoka, Nara, and Hokkaido) of Japan were selected for the survey. Average daily intake of food from 5 years before the diagnosis was measured by means of a semi-quantitative food frequency questionnaire. We studied 140 cases and 140 individually age ( ± ± ± ±5 years)-matched hospital controls for analysis. Estimates of age-adjusted odds ratios (ORs) and linear trends were calculated by conditional logistic regression models with adjustment for cigarette smoking and total energy intake as confounding factors. Consumption of fish, all soybean products, tofu (bean curds), and natto (fermented soybeans) was associated with decreased risk. ORs of the fourth vs. first quartile and 95% confidence intervals (95%CIs) were 0.45 (0.20-1.02) for fish, 0.53 (0.24-1.14) for all soybean products, 0.47 (0.20-1.08) for tofu, and 0.25 (0.05-1.24) for natto. Consumption of fish and natto showed significantly decreasing linear trends for risk (P < < < <0.05). Consumption of meat was significantly associated with increased risk (the OR of the second vs. first quartile was 2.19, 95%CI 1.00-4.81). Consumption of milk, fruits, all vegetables, green-yellow vegetables, and tomatoes showed no association. Our results provide support to the hypothesis that the traditional Japanese diet, which is rich in soybean products and fish, might be protective against prostate cancer. (Cancer Sci 2004; 95: 238-242) he age-standardized incidence of prostate cancer is low in Japan (10.0 per 100,000), being approximately one-tenth of that in the United States.1) Among Japanese immigrant in Hawaii, age-standardized incidence of prostate cancer is approximately ten times that in Japan.2) It is suggested that environmental factors such as dietary habits play a major role in causation of prostate cancer. A number of case-control studies have indicated that high intake of fat, especially saturated fat, increases the risk of prostate cancer.3) In contrast, high intake of isoflavone, which is mainly found in soybeans and soy products, may reduce the risk of prostate cancer. 4,5) The traditional Japanese diet which includes many soy products is richer in isoflavone, 6,7) and contains less fat than the Western diet. However, an epidemiological study of prostate cancer in Kyoto in 1988 8) yielded no evidence that fat or soybeans were associated with the risk of prostate cancer.Recently in Japan, the incidence rate of prostate cancer has been increasing cocomitantly with the westernization of dietary habits. We carried out a case-control study on dietary factors and prostate cancer to assess the hypothesis that the traditional Japanese diet reduc...