A prospectively controlled randomized trial to compare the adjuvant efficacy of 12 cycles of cyclophosphamide, methotrexate, fluorouracil, prednisone, and tamoxifen (CMFPT) followed by observation or a total of 5 years of continuous tamoxifen versus four cycles of CMFPT followed by observation in postmenopausal women with operable node-positive breast cancer was started by the Eastern Cooperative Oncology Group (ECOG) in 1982. In 1986 the study was modified to allow patients on CMFPT X 12 plus continuous tamoxifen to be rerandomized after completing 5 years of tamoxifen to either continue for life or to stop therapy. Patients were stratified for number of involved nodes and estrogen-receptor (ER) status and randomized to receive one of three treatments: CMFPT X 4, CMFPT X 12, or CMFPT X 12 plus continuous tamoxifen. Of 962 patients entered on the study, 803 were eligible. Life-threatening toxicity occurred in 75 and lethal toxicity in seven patients. Median follow-up is 4.1 years; 279 patients had recurrent disease. Time to relapse (TTR) is significantly longer for patients on CMFPT X 12 plus continuous tamoxifen than for CMFPT X 4 (P = .002), or CMFPT X 12 (P = 0.05). Differences between four or 12 cycles of CMFPT are not significant; relapse-free rates at 5 years are 61% for CMFPT X 12 plus continuous tamoxifen, 51% on CMFPT X 12, and 52% on CMFPT X 4. Treatment differences in overall survival are not significant. Hormone receptor status and number of involved nodes were found to be significant prognostic parameters.