Breast preservation has been shown to be a good alternative to mastectomy in selected patients with breast cancer. The purpose of this study was to evaluate the characteristics of cancer developing in the opposite breast to determine if breast preservation should be attempted in that breast as well. From 1979 to 1988, 172 women underwent tylectomy, axillary dissection and irradiation for carcinoma of the breast. All had follow-up mammogram. Mean age was 55 years. Mean follow-up time was 50 months. Thirteen patients (7.6%) developed cancer in the opposite breast. Three cancers were carcinoma in situ, nine were stage I, and one was stage IIa. Nine of 13 patients had breast preservation therapy, and four had mastectomies. Ten patients are alive with no evidence of disease, two are alive with disease and one died with disease. Breast preservation for bilateral breast cancer is a safe alternative if patients can be followed closely.