Ninety-seven patients were entered into a prospective randomized study of 2 split course regimens of radiation therapy for inoperable carcinoma of the lung. Group A received a 3,000 rads course with a month’s interval followed by a like course; Group B received an initial course of 2,000 rads in 5 treatments followed by a month’s rest and repetition of the same course. The survival of the patients completing both segments of the split course was significantly better in Group A than in Group B at 12 months, but not at 18 months. The over-all survival, free of disease, at 24 months was 1 per cent. Further studies are needed to achieve a better definition of the time/dose relationships in split course therapy for carcinoma of the lung.