Although the majority of patients with early breast cancer treated with conservative surgery and radiotherapy have acceptable cosmetic outcomes, the specific causes of a less-than-acceptable result are not well known. To assess the technical factors associated with late cosmetic failure, we reviewed the records of 593 patients treated with conservative surgery and radiotherapy at the Joint Center for Radiation Therapy, Boston, between 1968 and 1981. Median follow-up was 76 months with a range of 37-186 months. Using a 4-point scoring system (excellent, good, fair, poor) patients were noted to have cosmetic "failure" if scored as either fair or poor. Cosmetic results were similar at 3, 5, and 7 years with excellent or good scores seen in 88%, 90%, and 78% of patients, respectively. Three technical factors were associated with a significant worsening of the cosmetic results. The resection of 70 cm3 or more of breast tissue was more common among the failed patients than among matched patients with good or excellent results (p = 0.03). Eighty-nine percent of patients treated with a tangent pair technique had excellent results at 5 years compared to 69% of patients treated with a three-field technique (p = 0.004). The use of a larger volume implant was associated with a greater risk of a fair or poor cosmetic result compared to the use of a smaller volume implant. We conclude that overall cosmesis has been acceptable in this series, but there are technical (surgery and radiotherapy) factors which can be modified to optimize the cosmetic result without compromising local tumor control.