In the period between September 1983 and July 1987, 28 locoregional recurrences (about 86% in the chest wall) of breast cancer in 19 patients were treated with external microwave hyperthermia. Of these, 11 lesions were treated with hyperthermia alone and 17 with the combination of heat and external irradiation. Hyperthermia was induced in most cases with 915 MHz microwave (at least 42 degrees C for 30-45 min, for 6-10 sessions). Radiation was administered using photon or electron beams with total doses varying from 2000 to 5000 cGy. Complete and partial response rates on the whole series of patients were 53.6% and 42.9%, respectively. Of 17 lesions treated with combined treatment, 10 had a complete (58.8%) and 7 a partial response; of 11 lesions treated with hyperthermia alone, 5 had a complete (45.5%), 5 a partial (45.5%), and 1 no response. Complete responses were long lasting in all but 3 cases, which recurred after 4 months (2 cases) and 1 year, respectively.