Twenty patients with extensive pelvic tumours have been treated with radiotherapy and regional hyperthermia using an annular phased array applicator (APA). The majority of patients had bulky disease with a mean tumour volume of 206 cm3. In 63 of 67 attempted treatments heating was achieved, temperatures in excess of 42 degrees C being reached in the pelvis in 78%. Heat doses, however, were low, only 41% of treatments exceeding 5 min equivalent at 43 degrees C. Treatment was usually limited by acute toxicity and, although this was marked, it was of short duration and no long-term sequelae have been noted. With gain in operator experience, later treatments have achieved improved thermal doses. Response was difficult to assess but we have documented responses in six of 12 assessable patients. Palliation has been achieved in all patients with symptoms. Preliminary attempts to shift the region of maximum heating by phase variations in the APA output are promising, and this technique should improve the quality of treatments and reduce toxicity. The APA is thus capable of inducing pelvic regional hyperthermia, and a randomised controlled study is now planned to assess the value of pelvic hyperthermia as a treatment modality.