Chemotherapeutic treatment for cancer has been successful in prolonging survival but may also lead to the development of second cancers. Two case-control studies presented here suggest, however, that breast cancer patients who receive chemotherapy are at significantly lower risk of a contralateral breast cancer than those who do not. Approximately 300 incident cases of contralateral breast cancer and 300 randomly chosen surviving controls with unilateral breast cancer were identified through the Connecticut Tumor Registry for inclusion in each study. The initial study was based on review of medical records at eight hospitals and indicated that the overall association with chemotherapy was modified by body build. The second study obtained information from in-person interviews, hospital records, and outpatient chemotherapy records from across the state. The preliminary results of this second study confirm the previous findings. Both cytotoxic and hormonal drugs were associated with a reduction in the risk of second breast cancers (OR = 0.5, 95 percent CI: 0.3-1.0; OR = 0.5, 95 percent CI: 0.2-1.2, respectively). Significant interaction with body build was observed for hormonal treatment (ratio of ORs = 5.8, 95 percent CI: 1.0-34.3 for a five-unit change in Quetelet's index), with a nonsignificant but detrimental effect suggested for overweight women (OR = 2.3, 95 percent CI: 0.4-13.9 for a Quetelet's score of 35).