From September 1987 to August 1990, 36 patients with stage III non-small cell lung cancer were randomly assigned to 65 Gy in 26 fractions of 2.5 Gy each (conventional radiotherapy) or 71.5 Gy in 52 fractions of 1.375 Gy given twice a day (hyperfractionated radiotherapy). The overall time for both groups was 6.5 weeks. The groups were similar in terms of age, performance status, previous chemotherapy, stage IIIA and IIIB, and histopathology. The rate of complete response was 44.4% for hyperfractionation, and 16.7% for conventional fractionation. Overall survival rates at 24 months and 36 months were 50.0% and 21.8% for hyperfractionation, and 31.3% and 0% for conventional fractionation. Fever due to radiation pneumonitis occurred in seven of 18 patients with hyperfractionation (38.9%) and four patients with conventional fractionation (22.2%). No severe late toxicity has been observed in either group. We conclude that hyperfractionated radiotherapy in non-small cell lung cancer may improve survival without increasing severe toxicity.