A long-term prospective study of non-infiltrating breast carcinoma is being carried out in order to study the natural history and proper management of such lesions with particular interest in patients treated solely by local excision. During an 11-year period, 175 patients with lobular Ca in situ and intraductal Ca have been followed. None has developed recurrent disease including 18 undergoing wide local excision only. Histologic examination revealed that 36% of mastectomy specimens showed multifocal lesions, whereas only one of the patients treated by wide local excision had more than one microscopic focus. For these reasons total mastectomy is recommended as the treatment of non-infiltrating breast carcinoma. Subsequent development of contralateral cancer has occurred in ten patients.