Bilateral breast cancer has a cumulative incidence of about 7% in patients with primary operable breast cancer, and most of these lesions are metachronous. Most retrospective studies have shown that a majority of these patients have invasive cancer in the second breast, and varying percentages have nodal metastases, which may be of a higher stage than the first cancer. Physicians are now more aware of the importance of careful monitoring of the second breast after ipsilateral mastectomy, and improvements have been made in mammographic surveillance. A retrospective, comparative analysis of two separate breast cancer populations at risk for bilateral breast cancer was done on patients who entered into the system before effective mammographic monitoring (BEM) and after effective mammographic monitoring (AEM). The first group of patients consisted of 500 consecutive patients with primary breast cancer diagnosed during the years 1969 through 1975, of whom 37 (7.4%) had bilateral breast cancer. The second group consisted of 557 consecutive patients diagnosed during the years 1977 through 1984, of whom 36 (6.5%) had bilateral breast cancer. The staging percentages of the second breast cancer in the BEM group were Stage 0, 5.4%; Stage I, 48.6%; Stage II, 10.8%; Stage III, 21.6%; and Stage IV, 13.5%. The second group had an improvement in stage, with 33.3% being Stage 0, 22.2% Stage I, 29.6% Stage II, 3.7% Stage III, and 3.7% Stage IV (P less than 0.05). The median interval between primary lesions was 39 months in the first group and 19 months in the second group (in part, this difference may represent increased identification of synchronous cancers). The second breast cancer was undetected by mammography in 9 of 34 (26%) patients. Six were detected by contralateral biopsy (all were lobular carcinomas in situ), and three were found by clinical examination (all were invasive cancers). It was concluded that more aggressive monitoring of the second breast by frequent clinical examination, mammography, and selected contralateral biopsy appears to have increased the early detection rate of second breast cancers in patients under observation.