Second primary breast cancer is of importance because of the increasing incidence of first primary breast cancer and improved survival. There is lack of information on the disease in Canada. We studied 14,220 women with a first primary breast cancer (invasive only) diagnosed during the period from 1970 to 1997, in Manitoba, Canada. A breast cancer was considered to be a second primary if it was an invasive lesion diagnosed at least six months after the diagnosis of an initial primary breast cancer. The incidence rate of second primary breast cancer was approximately six per 1,000 person-years, and the cumulative incidence during the 28-year study period was 14.3%. Average age at diagnosis of first primary breast cancer had increased significantly since 1970. The incidence was higher in 1975-1979, and there was no significant difference among other study periods after adjustment for the age at diagnosis of first primary breast cancer. Standardized incidence ratios showed no significant difference among six age groups after adjusting for calendar year. However, women with a first primary breast cancer had an increased risk of developing a second primary breast cancer compared with the risk of developing a first primary breast cancer among the general female population. The SIR was markedly greater among women who had a first primary breast cancer before the age of 40 years, and decreased significantly with increasing age at diagnosis of first primary breast cancer. The data provide epidemiological information on second primary breast cancer risk for the general female population in Manitoba, Canada.