Three hundred ten patients with Stage II or Stage III breast cancer were entered on an adjuvant protocol consisting of a combination of 5-fluorouracil, doxorubicin, cyclophosphamide, vincristine, and prednisone (FACVP). In the second phase of the study, patients with estrogen receptor-negative tumors received sequential courses of methotrexate and vinblastine. Other patients, who were estrogen receptor-positive or unknown, were randomized to receive either tamoxifen alone or tamoxifen plus methotrexate and vinblastine. All therapy was completed within 1 year. The estimated disease-free rate at 5 years was 68% among patients with Stage II disease and 52% for patients who had Stage III disease. Among patients with estrogen receptor-positive tumors, disease-free survival was significantly prolonged in patients who received methotrexate and vinblastine in addition to tamoxifen (P = 0.04). However, this difference was less pronounced when all randomized patients (including those whose estrogen receptor status was unknown) were included in the comparison. Although most patients experienced moderate to severe granulocytopenia, infectious complications were infrequent. One patient died of septicemia. Congestive heart failure developed in two patients, one of whom had a history of myocardial infarction and congestive heart failure.