The epidemiologic features of breast cancer in Rochester, Minnesota, were studied over the 40-year period 1935 to 1974. During this time, the age-adjusted incidence rate increased 25% to 82.7 per 100,000 person-years. Most of the increase occurred among women 45 to 64 years of age. Overall, the incidence of breast cancer increased with advancing age, as did breast cancer prevalence, which was 901.4 per 100,000 women on Jan. 1, 1975. Age-adjusted mortality rates declined slightly from 24.9 to 23.3 per 100,000 person-years during the study period. Survival after the diagnosis of breast cancer seemed to improve somewhat among women who were less than 45 or more than 65 years old at the time of diagnosis. This was associated with a decline in the proportion of these women with regional lymph node involvement or distant metastases present at the time of initial diagnosis.