Between 1953 and 1973 the mean incidence of a first breast cancer in northern Alberta was 0.47 per 1000 women. However, in women who had had a first breast cancer the crude incidence of a second primary cancer in the opposite breast was 1.3%. The rate of discovery of a second primary cancer increased after a multidisciplinary approach was instituted at the follow-up clinics. Mammography has proved to be valuable in diagnosis. By this technique 19 (36.5%) of the 52 cancers occurring in the remaining breast were identified when they were clinically unsuspected.