A clinicopathologic study of atypical lesions of the breast further follow up.

On the assumption that both the lobular and intraductal atypical lesions of the breast are precancerous in nature, a group of 572 such patients have been closely followed during the past 12 years. Thirty-four patients eventually developed cancer during this period. The cumulative incidence of breast cancer was 7.74% at 10 years in the group of patients with atypical lesions only and 16.15% at 10 years in the patients with atypical lesions in one breast and cancer in the contralateral breast. The transformation of this probable precancerous state into cancer apparently takes a long interval of time.