Between May, 1963 and December, 1966, 17 medical centers cooperated in two separate but integrated therapeutic trials of primary lung cancer. One study was of patients with lesions considered operable at the time of diagnosis, and the other of patients with initially inoperable cancer but who were considered potentially operable after radiotherapy. Patients operable at the time of diagnosis were randomly assigned to receive either immediate surgery (278 patients) or preoperative radiotherapy followed by surgery (290 patients). All but one were followed until death or 5 years survival. Survival to each anniversary after randomization was almost identical for the two groups. At 5 years the survival rate was 14% after preoperative radiotherapy and 16% after immediate surgery. On the basis of the small standard error of the difference between these survival rates, a large advantage or a large disadvantage for preoperative radiotherapy is unlikely. Recurrence of cancer either locally or as distant metastasis was also similar in the two groups. Postoperative mortality was estimated to be 11% in the immediate surgery group, but cannot be estimated in a comparable fashion for the irradiated group. Certain postoperative complications were more frequent in the irradiated group, but survival during the first was not affected. Out of 425 patients initially considered to be inoperable, 152 were considered resectable after radiotherapy. These patients were randomly assigned to have either a thoracotomy and resection of their cancer if possible (78 patients) or no surgery (74 patients). Survival to each anniversary after randomization was very similar. After 5 years the survival rate was 8% for the group assigned to surgery and 6% for the group assigned to no surgery. The difference has a standard error of 4%.