We have retrospectively reviewed 112 cases of in situ carcinoma of the breast treated between 1960 to 1972, with a minimum 10 year follow-up to correlate treatment with outcome. We concluded that bilateral simple mastectomy with low axillary dissection is the treatment of choice for intraductal or lobular carcinoma in situ. This conclusion was based on the early age at diagnosis, the high incidence of bilaterality, and the long latency and probable progression from the in situ stage to the invasive stage. Lesser procedures can be endorsed for those patients of advanced age or who have associated medical problems whose life expectancy is estimated to be 10 years or less. Patients who refuse bilateral mastectomy should undergo biopsy of the involved or opposite breast at 3 to 5 year intervals regardless of physical findings or mammographic suspicions, especially when severe epithelial dysplasia is encountered in the biopsy specimens. This nonpalpable but potentially curable lesion remains difficult to detect even by mammography.