PURPOSE: Adjuvant tamoxifen has been shown to reduce relapse and mortality among node-positive post-menopausal breast cancer patients. The value of adding chemotherapy to tamoxifen is controversial. PATIENTS AND METHODS: Between July 1986 and April 1993, 1,266 postmenopausal breast cancer patients with node-positive disease were randomly assigned to receive one of four adjuvant therapy regimens: (A) tamoxifen alone for 5 years; (B) tamoxifen plus three courses of early cyclophosphamide, methotrexate, and fluorouracil (CMF) on months 1, 2, and 3; (C) tamoxifen plus delayed single courses of CMF on months 9, 12, and 15; (D) tamoxifen plus early and delayed CMF on months 1, 2, 3, 9, 12, and 15. The two-by-two factorial design allowed two direct comparisons: early CMF (B and D) versus no early CMF (A and C), and delayed CMF (C and D) versus no delayed CMF (A and B). Estrogen receptor (ER) status was known for all patients and was used to stratify the randomization. A total of 1, 212 patients (96%) were eligible and assessable. The median follow-up duration was 60 months. RESULTS: The results of the two-by-two factorial comparisons were as follows: (1) early CMF added to tamoxifen significantly improved 5-year disease-free survival (DFS; 64% v 57%; hazards ratio [HR], 0.79; 95% confidence interval [CI], 0.66 to 0.95; P = .01); and (2) delayed CMF added to tamoxifen did not improve DFS (5-year DFS, 61% v 60%; HR, 0.97; 95% CI, 0.81 to 1.17; P = .77). For patients with ER-positive tumors, the addition of CMF, either early or delayed or both, reduced the relative risk of relapse by 22% to 36%. In contrast, for patients with ER-negative tumors, tamoxifen with delayed CMF was associated with a nonsignificant increased risk of relapse (HR, 1.27; 95% CI, 0.92 to 1.76; P = .15). CONCLUSION: Postmenopausal patients with node-positive breast cancer should be offered combination chemotherapy in addition to tamoxifen. Tamoxifen should not be initiated before CMF, as this might be detrimental, especially for patients with ER-negative tumors.