Epidemiological characteristics of breast cancer have been studied in Tokyo, an area of relatively low breast cancer rates, as part of an international collaborative study. Breast feeding of infants is customary for Japanese women, but even very prolonged lactation did not appear to be associated with a reduction in breast cancer risk. Late age at menarche and early age at first pregnancy were both associated with reduced breast cancer risk. The protective effect of early pregnancy seems much greater than has previously been recognized, and may indeed be greater in Japan than in other areas. However, the average age at first pregnancy is not lower in Tokyo than in areas than in areas where breast cancer rates are high.