The acute toxicity of an accelerated radiotherapy scheme was compared with a conventional schedule. Eighteen patients with squamous cell carcinoma of the larynx were treated with accelerated fractionation radiotherapy. An average reduction of overall treatment time of 11 days was accomplished by giving 2 fractions a day during the last part of the treatment. Total dose and fraction dose were left unchanged. Acute reactions of skin and mucosa in these patients were compared with those in 40 patients treated with a conventional fractionation scheme, that is, 2 Gy per fraction, 5 fractions per week, to a total dose of 64-70 Gy. Acute reactions were maximal between 5 and 7 weeks after the start of treatment. Complete healing occurred within 3 months in all patients. Mucosal reactions and, as a consequence, dysphagia were clearly increased in those patients treated with accelerated fractionation. For confluent mucositis an ED50 of 66 Gy was calculated compared to 69 Gy for conventional fractionation. To a lesser degree, skin toxicity was also enhanced in the patients treated with the accelerated schedule. Severe edema of the laryngeal mucosa occurred only in patients treated to a total dose of 68 or 70 Gy and was somewhat more frequent with accelerated fractionation (4/10) than with conventional fractionation (4/24). One patient in the accelerated fractionation group underwent laryngectomy for persistent edema and laryngeal necrosis. No severe late skin reactions were observed. It can be concluded that the fractionation schedule tested in this study is feasible. Further shortening of overall treatment time without reduction of total dose will likely lead to unacceptable acute and, possibly, also late toxicity.