The incidence rate of breast cancer in Japan rose more than two-fold from 1959-60 to 1983-87. To assess to what extent this increase can be explained by changes in the prevalence of four major risk factors of breast cancer (i.e, age at menarche, age at first birth, age at menopause, and parity), we estimated the probability of developing breast cancer based on the joint distribution and relative risks of these four risk factors. The age-specific incidence rate during 1959-60 reported by the Miyagi Prefectural Cancer Registry was used to estimate the baseline hazard rate for women without the four risk factors in the same age group. Assuming that the baseline hazard rate is constant during all periods, we calculated the expected incidence rates during the periods of 1959-60, 1962-64, 1968-71, 1973-77, 1978-81, and 1983-87 for each age group. Large discrepancies were noted between the observed and expected incidence rates during 1983-87 in all age groups. The change in the joint distribution of the four risk factors accounted for less than 40% of the increase observed from 1959-60 to 1983-87, suggesting the effects of other powerful risk factors.