The association between soy product intake and the occurrence of hot flashes was examined in a cohort of 1,106 female residents of Takayama, Gifu, Japan. The women were aged 35-54 years and premenopausal at their entry into the study in 1992. Diet, including intake of soy products and isoflavones, was assessed by means of a validated semiquantitative food frequency questionnaire at study entry. A follow-up mail questionnaire asking about experiences of hot flashes was sent in 1998. During the 6 years of the study period, 101 women had new moderate or severe hot flashes according to the Kupperman test of menopausal distress. After data were controlled for age, total energy intake, and menopausal status, hot flashes were significantly inversely associated with consumption of soy products in terms of both total amount (highest tertile of soy product intake (g/day) vs. lowest: hazard ratio = 0.47; 95% confidence interval: 0.28, 0.79; p for trend = 0.005) and isoflavone intake (highest tertile of isoflavone intake (mg/day) vs. lowest: hazard ratio = 0.42; 95% confidence interval: 0.25, 0.72; p for trend = 0.002). These data suggest that consumption of soy products has a protective effect against hot flashes.