A retrospective review of the outcome of treatment for primary, Stage I and II breast cancer with segmental mastectomy (SGM) alone or segmental mastectomy plus postoperative irradiation (SGM + RT) at four Rochester, New York, city hospitals is reported. Between January 1971 and March 1984, 99 women were treated with SGM and 146 with SGM + RT. Groups were similar regarding significant clinical and histologic prognostic factors; they differed, however, in that the SGM group was considerably older (means = 72) than the SGM + RT group (means = 56). Among SGM patients, local and total locoregional failure was 26.44 and 35.2%, respectively. Local and total locoregional failure (7.6 and 12.4%, respectively) was significantly reduced among patients treated with SGM + RT (p less than 0.0001). Among SGM patients, there was scant advantage in enlarging the extent of resection from local excision (29.5% local failure) to wide local excision (27.3%) to quadrantectomy (22.2%). Among women receiving SGM + RT, similar rates of local failure occurred among patients receiving local excision (15.5%) and wide local excision (12.5%). By contrast, only 2.8% of those receiving quadrantectomy failed. Results are viewed as supportive of findings of NSABP-B06. Findings suggest that SGM constitutes inadequate treatment of Stage I and II breast cancer. Locoregional failure rates of 30-40% may be reduced to around 10% with postoperative irradiation.