PURPOSE: To determine whether preoperative doxorubicin and cyclophosphamide (AC) permits more lumpectomies to be performed and decreases the incidence of positive nodes in women with primary breast cancer. PATIENTS AND METHODS: Women (n = 1,523) were randomized to National Surgical Adjuvant Breast and Bowel Project (NSABP) B-18; 759 eligible patients received postoperative AC and 747, preoperative AC. The clinical size of breast and axillary tumors was determined before each of four cycles of AC and before surgery. Tumor response to preoperative therapy was clinically complete (cCR), partial (cPR), stable (cSD), or progressive disease (cPD). Tissue from patients with a cCR was evaluated for a pathologic complete response (pCR). RESULTS: Breast tumor size was reduced in 80% of patients after preoperative therapy; 36% had a cCR. Tumor size and clinical nodal status were independent predictors of cCR. Twenty-six percent of women with a cCR had a pCR. Clinical nodal response occurred in 89% of node-positive patients: 73% had a cCR and 44% of those had a pCR. There was a 37% increase in the incidence of pathologically negative nodes. Before randomization, lumpectomy was proposed for 86% of women with tumors < or = 2 cm, 70% with tumors 2.1 to 5.0 cm, and 3% with tumors > or = 5.1 cm. Clinical tumor size and nodal status influenced the physician's decision. Overall, 12% more lumpectomies were performed in the preoperative group; in women with tumors > or = 5.1 cm, there was a 175% increase. CONCLUSION: Preoperative therapy reduced the size of most breast tumors and decreased the incidence of positive nodes. The greatest increase in lumpectomy after preoperative therapy occurred in women with tumors > or = 5 cm, since women with tumors less than 5 cm were already lumpectomy candidates. Preoperative therapy should be considered for the initial management of breast tumors judged too large for lumpectomy.