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From March 1981 to June 1989, 44 patients with locally recurrent, previously irradiated adenocarcinoma of the breast were treated with hyperthermia and irradiation. Treatment sites were chest wall (35) or nodal (nine), with an average tumor area of 29.44 cm2 (range 0.16 to 252 cm2) prior to treatment. Concurrent radiation doses varied from 16 to 56 Gy (mean = 29.4 Gy). Externally applied microwave hyperthermia was given twice weekly, aiming at 43 degrees C for 60 min. Evaluation at one month post treatment revealed 41% complete response (CR), 23% partial response (PR), and 36% no response (NR), and 67% of patients with a CR sustained that response for greater than 12 months. Tumors heated to a mean thermal dose (equivalent-minutes at 42.5 degrees C) greater than 50 had a 53% CR rate, significantly better than the 14% CR rate observed in patients whose tumors received a mean thermal dose less than 50. Among patients with tumors less than or equal to 6 cm2 in area, 65% achieved CR, significantly better than the 26% CR rate noted for patients with tumors greater than 6 cm2 in area. Only four patients (7.4%) experienced complications: one developed a catheter-related infection, two had ulcerated infections, and one had a severe blister. In summary, higher thermal doses delivered and smaller tumor areas were associated with more favorable tumor responses.