A retrospective study was undertaken of weight gain during the first year after diagnosis of breast cancer in three groups of women with localized disease. Group I consisted of 307 women with clinical or pathological node-negative breast cancer and Group II of 139 women with clinical or pathological node-positive breast cancer. Neither of these groups received systemic adjuvant therapy. Group III consisted of 191 women with pathological node-positive breast cancer who received adjuvant cyclophosphamide, methotrexate, and 5-fluorouracil (CMF) (Group IIIA) or CMF plus prednisone (7.5 mg per day) and ovarian ablation (Group IIIB). In the absence of systemic therapy, weight gain was greater in those with involved nodes than in those with uninvolved nodes (p = 0.001). When women with involved nodes were studied, those who received prednisone and ovarian ablation in addition to CMF gained more weight than those who received CMF alone (p less than 0.0001). The use of CMF alone did not significantly affect weight gain. Neither initial weight nor weight gain after diagnosis affected prognosis. It was concluded that the observed weight gain was not due to the use of adjuvant CMF and did not affect prognosis. Further investigation was recommended to replicate the above findings and to determine the cause of weight gain after diagnosis in women with localized breast cancer.