Racial differences in the incidence of prostate cancer are manifest worldwide, possibly due to the different dietary habits. To elucidate the relationship between the recent trend of phytoestrogenic isoflavone intake and the increased incidence of prostate cancer in Japan, we conducted an age-stratified dietary survey of soybean foods in 102 Japanese healthy men (age range: 10-59 years) and measured the serum isoflavones and equol levels in them and 100 Korean healthy men. The intergroup comparison among the age-stratified groups showed significant differences in the daily intake of genistein and daidzein between the teenager group and the other groups of age >or=30 years (P<0.05). In the Japanese study, the proportion of equol producers in the teenager group was 10%, being significantly the lowest among the age-stratified groups. The proportions of equol producers in the age-stratified groups from 10 to 49 years were also significantly lower than those in the fifties. The equol non-producers consumed significantly less amounts of isoflavones than the equol producers. In the Korean study, the proportions of equol producers were 45% in the teenager and 40% in the twenties and thirties, being significantly lower than in the forties (80%) and fifties (65%). The decreased intake of isoflavones, low serum level of equol and low incidence of equol production in the young generation may become potential risk factors for prostate cancer not only in Japan but also in Korea in the near future. Elucidating the mechanism of equol production may be promising in developing strategies for chemoprevention against prostate cancer.