AbstractFrom August 1971 to August 1974, almost 1700 patients with clinically curable breast cancer in 34 institutions in the United States and Canada were entered into the National Surgical Adjuvant Breast Project (NSABP) trial comparing the results of total (simple) mastectomy with those of radical mastectomy. Patients with clinically negative axillary nodes in the total mastectomy group were further randomized to receive postoperative regional radiotherapy or no radiotherapy, but the latter patients underwent an axillary dissection if clinically positive axillary nodes developed after operation. Patients with clinically positive axillary nodes in the total mastectomy group automatically received postoperative regional radiotherapy. After a follow‐up period of 2 to 5 years, there are no significant differences in the results of the various types of treatment in either the groups of patients with clinically negative axillary nodes or those with clinically positive axillary nodes.In June 1976 the NSABP initiated a new study comparing total mastectomy plus axillary dissection, segmental mastectomy plus axillary dissection, and segmental mastectomy plus axillary dissection plus postoperative radiation of the involved breast. All patients with histologically positive axillary nodes receive adjuvant chemotherapy. In the two segmental mastectomy groups, subsequent appearance of cancer in the involved breast is treated by total mastectomy.