We evaluated the use of regional hyperthermia with radio-chemotherapy in a phase I/II study on locally advanced rectal carcinomas. Thirty-four patients with primary advanced (stage T3/T4) rectal carcinomas (24 patients) or recurring rectal carcinomas (6 patients) were treated using preoperative radiochemo-thermotherapy. Initial tumour staging was carried out clinically (degree of fixation) and using endorectal ultrasonography and CT. Radiotherapy was carried out with the patient prone (on a belly board) at 5 x 1.8 Gy per week up to 45 Gy at the reference point. 5-Fluorouracil (300-500 mg/m2) was administered with low-dose leucovorin (50 mg) on days 1-5 and 22-26. Patients were treated with regional hyperthermia each week prior to radiotherapy, using the Sigma-60 ring of the BSD-2000 system. Temperature/position curves and temperature/time curves were recorded via endocavitary catheters (tumour contact, bladder, vagina). Following endosonographic and clinical restaging, the operation was carried out 4-6 weeks after the end of preoperative therapy. In cases where tumours were unresectable, a boost of up to 60 Gy was given. Twenty-three of the 34 patients (68%) proved to be curatively resectable. Of these patients, 70% were downstaged endosonographically during preoperative therapy. The actuarial survival rates among these patients were 85% (primary rectal cancer) and 60% (recurrences) at 30 months. All in all, the preoperative multimodal therapy was well tolerated, and premature termination was necessary in only two cases. The quality of temperature distribution (T90, cum min T90 > 40.5 degrees C) depends on the power level and relative power density. The response (particularly downstaging) correlates significantly with the quality parameters of the temperature distributions. This regimen proved practical and effective, with encouraging downstaging rates and local control rates.