Sixty-nine chest CT studies of 36 patients with lung cancer were obtained at various times (21 to 649 days) following completion of fast neutron radiotherapy. Multiple scans (2 to 7) were available in 16 cases. Fractionated neutron radiotherapy was delivered over four to seven weeks to a total dose of 60 to 67 Gy photon equivalent. Parenchymal change within the lung was noted in 43% by 12 weeks, in 91% between 12 and 24 weeks, and in 100% after 24 weeks. Three different patterns of lung parenchymal change were evident. Ectatic air-filled bronchi were observed in more than 50% after 24 weeks. Mediastinal shift and thoracic shrinkage were demonstrated in six and three cases, respectively. Serial CT scans revealed that lung parenchymal changes usually became stable or shrunk after 36 weeks. The possible effect of chemotherapy (mitomycin-C, vinblastin and cis-platinum), in 19 of these patients is also discussed.