OBJECTIVE The current sheet applicator (CSA) is a newly developed microwave hyperthermia device. Advantages over commercial microwave applicators include its small size and high ratio of heating area to physical aperture area. These physical characteristics make the CSA excellent for heating constricted areas and allow the use of arrays of CSAs over large surfaces. This study examines the clinical efficacy of the CSA for heating superficial malignant tumors. METHODS From December 1989 through October 1991, 19 patients with recurrent or metastatic superficial malignant tumors were treated once or twice weekly to 30 hyperthermia fields using one to four CSAs. Each field received from one to four hyperthermia treatments for a total of 74 treatments. The treatment objective was to elevate the tumor temperature to a minimum of 42.5 degrees C for 30 min (2 patients) or 60 min (17 patients). Intratumor temperatures were measured with percutaneous fiberoptic thermometry probes. All patients received concurrent fractionated radiation therapy with total dose ranging from 20 to 65 Gy (median 46 Gy). Seventeen of the 30 fields had been previously irradiated to a median dose of 50 Gy. RESULTS Mean values for the maximum temperature, average temperature, and minimum temperature were 43.6 degrees C +/- 1.0, 42.2 degrees C +/- 1.4, and 41.0 degrees C +/- 1.5, respectively. Mean values for T50 and T90 were 42.2 degrees C +/- 1.1 and 41.0 degrees C +/- 1.3, respectively. The overall response rate for all assessable fields was 96%. Only Only three responding tumors have progressed with a median follow-up period of 6 months. Treatment related morbidity was generally mild and self-limited. CONCLUSIONS The CSA is a promising new microwave hyperthermia device capable of heating superficial tumors to therapeutic temperatures. When used in combination with radiotherapy, response rates are excellent without excessive toxicity.