A fluorinated 2-nitroimidazole radiosensitizer KU-2285 was given before intraoperative radiotherapy (IORT) to 30 patients with unresectable, unresected or macroscopic residual tumours. Twenty-three patients had pancreatic cancer and five had osteosarcoma. The IORT dose was 30 Gy for unresectable pancreatic cancer and 60 Gy for osteosarcoma. The dose of KU-2285 administered ranged from 1 to 9 g m-2. Four patients received a dose of 9 g m-2, and ten received 6.8-7 g m-2. All patients tolerated KU-2285 well, and no drug-related toxicity was observed. The average tumour concentration of KU-2285 immediately after IORT was 166 microg g-1 at dose of 6.8-7 g m-2 and 333 microg g-1 at 9 g m-2. The average tumour-plasma ratio was > or = 0.82. Eleven patients with unresectable but localized pancreatic cancer treated with KU-2285 plus IORT and external beam radiotherapy had a median survival time of 11 months and 1-year local control rate of 50%, which compares favourably with those of 8 months (P = 0.26) and 28% (P = 0.10) for 22 matched historical control patients. The five patients with osteosarcoma attained local control. The results of this first study on KU-2285 and IORT appear encouraging, and further studies of this compound seem to be warranted.