Data are presented on 1,244 patients who had subcutaneous mastectomies during the past fourteen years with an average follow-up of seven years. Obscure lobular carcinoma in situ was found in 4.3% of patients and obscure ductal or intraductal carcinoma was found in 5.1%, for a total of 9.4%. Breast cancer developed in only 6 patients after subcutaneous mastectomy, an incidence of 0.5%, indicating that subcutaneous mastectomy is effective in preventing breast cancer. Where obscure breast cancer was found, the prevalent associated benign fibrocystic diseases were microcystic and macrocystic and lobular hyperplasia. Where obscure ductal or intraductal carcinoma was found, the prevalent associated benign diseases were microcysts, macrocysts, intraductal hyperplasia, and sclerosing adenosis.