Urban-rural differential in breast cancer incidence and mortality in Olmsted County, Minnesota, 1935-1974.

Between 1935 and 1974, 580 new cases of breast cancer were diagnosed among the residents of Rochester, and 187 cases were found among the women living in the remainder of Olmsted County, Minnesota. The age-adjusted incidence of breast cancer was 79.1 per 100 000 person-years among Rochester women and 56.3 per 100 000 person-years among the rural women, for an urban-rural ratio of 1.40:1. The rural incidence increased more rapidly than did the urban incidence during the period of the study, especially among women 50 years old or older; and the urban-rural ratio seemed to decline with time. No convincing explanation for these changes was found, but migration and increasing urbanisation of rural areas may be important factors. The greater urban incidence and the net migration of rural patients with breast cancer into the urban area explain the higher urban mortality rates for breast cancer.