A prospective, randomized clinical study on 91 patients with squamous cell carcinoma of the oesophagus was undertaken in order to investigate the radiosensitizing effect of misonidazole. After histologic verification and extensive diagnosis, the greater tumor region was at first irradiated during 2.5 weeks with ten fractions of 3 Gy each up to a target volume dose of 30 Gy. Prior to each fraction, patients received randomly misonidazole or a placebo in a dose of 1 g/m2 body surface. Then they were presented to the oncologic surgeon in order to decide whether a surgical resection should be performed or not. Following to this operation no further radiotherapy was performed. However, if a surgical intervention did not take place, radiotherapy was continued without administration of misonidazole or placebo up to a target volume dose of 60 to 70 Gy. There was no evidence of neurotoxic side effects or modifications of the blood count and some laboratory parameters caused by misonidazole. As to recurrence-free interval and survival time, no significant differences were found between the different therapy groups, so that a radiosensitizing effect of misonidazole was not demonstrated in this study. Regarding several positive phase II studies with misonidazole, some hopes had been placed in this study because at present the therapeutic situation in oesophagus carcinoma is extremely unsatisfactory. Even the combination of a most sophisticated operation technique prior or following to irradiation could not essentially improve the poor healing rates.