A total of 285 patients with medically inoperable (RTOG Stage T1-2, N0-1) or unresectable (RTOG Stage T3, N0-1) non-small cell carcinoma of the lung were randomized by the Radiation Therapy Oncology Group (RTOG) to receive radiation therapy (6000 cGy total dose/6 weeks) plus levamisole (2.5 mg/kg twice weekly for 2 years or until tumor progression) or a placebo. One hundred twenty-nine evaluable patients were assigned to placebo and 131 to levamisole. This report is based on 260 (91%) eligible patients who started treatment and have adequate follow-up. Fifty percent of the patients in both treatment groups had Karnofsky scores of 90-100; 72% had squamous cell carcinoma, 12% adenocarcinoma, and 16% large cell undifferentiated carcinoma; 60% had RTOG Stage I or II primary tumors and 40% had Stage III (T3, N0-1) tumors. Complete regression of tumor was reported in 20% of the patients treated with levamisole and 36% of those receiving placebo. An additional 33% and 19%, respectively, had a partial response (trend test p = 0.08). Median survival was 9 months for patients treated with levamisole and 12 months for those on placebo (two-sided p less than 0.01); at 2 years, patients treated with levamisole had a 15% survival rate as compared to 24% in those receiving placebo. The cumulative proportion failing within the irradiated field with or without other sites of progression at 2 years was 30% in the levamisole group and 34% in the placebo patients. Median progression-free survival was 6 months for patients on levamisole and 7 months for those on placebo (overall two-sided p = 0.014); the estimated proportions progression-free at 2 years were 11% and 18%, respectively. The study showed no significant prolongation of survival, progression-free survival, or differences in patterns of failure in irradiated patients treated with levamisole compared with a placebo. Toxicity related to this immunoadjuvant was, in general, of moderate clinical importance. This study confirms a report by the Southeastern Cancer Study Group concluding that levamisole combined with definitive irradiation has no benefit in the treatment of unresectable non-small cell carcinoma of the lung.