In this trial, 381 women with invasive breast cancer in histopathologic stage I had sector resection with meticulous control for complete excision of local disease plus axillary dissection. We then randomly assigned 187 women to receive postoperative radiotherapy to a total mean dose of 54 Gy to the breast and 194 women to receive no further treatment. Criteria for eligibility were (a) a unifocal tumor 20 mm or less in diameter that was visible on a mammogram and (b) negative axillary lymph nodes on histopathologic examination. The mean follow-up time was 32.9 +/- 2.2 (SD) months in the group of patients treated with radiation and 29.3 +/- 2.4 months in the group treated with surgery alone. The actuarial local recurrence rate after 3 years was 2.9% (95% confidence interval, 0.1%-5.8%) in the group treated with radiation and 7.6% (95% confidence interval, 3.0%-12.3%) in the group treated with surgery alone (P = .06). The two treatment groups did not differ significantly in overall survival (P = .49) or survival free from regional or distant recurrence (P = .18). We conclude that radiation therapy saves some patients from local recurrence. However, our data suggest that, for women with small unifocal tumors without axillary lymph node involvement, a meticulous surgical technique may be an alternative to surgery plus routine radiotherapy.