The optimal duration and timing of adjuvant chemotherapy for breast cancer patients remain uncertain and were prospectively studied. We randomly assigned 1,554 premenopausal breast cancer patients with node-positive disease in a 2 x 2 factorial design to receive the following: (A) cyclophosphamide, methotrexate, and fluorouracil for 6 consecutive courses on months 1 to 6 (CMF x 6); (B) CMFx6 plus three single courses of reintroduction CMF given on months 9, 12, and 15; (C) CMF for three consecutive courses on months 1 to 3 (CMFx3); or (D) CMFx3 plus three single courses of reintroduction CMF given on months 6, 9, and 12. Accrual was between July 1986 and April 1993. A total of 1,475 patients (95%) were eligible and assessable. The median follow-up duration was 60 months. Patients who received CMFx3 without reintroduction had a 5-year disease-free survival (DFS) rate of 53% compared with 58% for the other three treatment groups (hazards ratio HR, 1.20; 95% confidence interval CI, 1.00 to 1.45; P = .04). The increased risk of relapse with CMFx3 was more marked for women aged less than 40 years (308 patients; HR, 1.32; 95% CI, 0.94 to 1.84; P = .11) and for patients with estrogen receptor (ER)-negative tumors (455 patients; HR, 1.45; 95% CI, 1.06 to 2.00; P = .02). Reintroduction chemotherapy provided additional benefit (HR, 0.86; 95% CI, 0.73 to 1.01; p = .07), especially for women > or = 40 years of age (1,167 patients; HR, 0.82; 95% CI, 0.68 to 0.99; P = .04). Three courses of adjuvant CMF chemotherapy are not sufficient compared with longer duration CMF chemotherapy, especially in younger women and in patients with ER-negative primary tumors. Reintroduction chemotherapy showed some evidence of additional benefit, but remains investigational.