The frequency of brachial plexopathy, rib fracture, tissue necrosis, pericarditis, and second non-breast malignancies occurring in the treatment field among 1624 patients with early stage breast cancer treated with conservative surgery and radiation therapy at the Joint Center for Radiation Therapy between 1968 and 1985 is reported. The median follow-up time for survivors was 79 months (range 5-233 months). Brachial plexopathy was related to the use of a third field, the use of chemotherapy and the total dose to the axilla. Brachial plexopathy developed in 20 of 1117 women (1.8%) who received supraclavicular irradiation with or without axillary irradiation. The median time to its occurrence was 10.5 months (range 1.5-77 mo), and the majority (80%) of cases completely resolved. Among patients treated with a three-field technique, the incidence of brachial plexopathy was 1.3% (13/991) in patients treated with a dose to the axilla of less than or equal to 50 Gy, compared with 5.6% (7/126) in women treated with an axillary dose of greater than 50 Gy. The incidence of brachial plexopathy was 4.5% (15/330) among patients receiving chemotherapy, compared with 0.6% (5/787) when chemotherapy was not used (p less than 0.0001). Rib fracture was seen in 29 patients (1.8%), at a median time of 12 months following treatment (range 1-57). In all cases, the rib fracture healed without intervention. The incidence of rib fracture was 2.2% (28/1300) among patients treated on a 4 MV linear accelerator, compared with 0.4% (1/276) for patients treated on a 6 or 8 MV machine (p = 0.05). Of patients treated on a 4 MV machine, 0.4% (1/279) developed a rib fracture when a whole breast dose of 45 Gy or less was given, 1.4% (10/725) after receiving between 45 and 50 Gy, and 5.7% (17/296) following 50 Gy or higher. Tissue necrosis requiring surgical correction developed in three patients (0.18%) 22, 25, and 114 months after treatment. Presumed pericarditis (requiring hospitalization) was seen in 0.4% of women (3/831) who received radiation therapy to the left breast 2, 2, and 11 months after the start of treatment. Three women (0.18%) developed sarcomas in the treatments field at 72, 107, and 110 months, for a 10-year actuarial rate of 0.8%. Two of these sarcomas developed in areas of probable match-line overlap. One patient (0.06%) developed an in-field basal cell carcinoma at 42 months. In conclusion, the risk of significant complications following conservative surgery and radiation therapy for early stage breast cancer is low.(ABSTRACT TRUNCATED AT 400 WORDS)