We retrospectively analyzed the risk of intracranial recurrence of cancer in 1532 patients who were surgically treated between 1977 and 1986 for Stage I, II, or III non-small-cell lung cancer, after rigorous surgical and pathological staging. This analysis was undertaken as a background for a possible randomized clinical trial of prophylactic cranial irradiation in such patients. One hundred four patients (6.8 percent) had documented first recurrences involving the brain, including 98 patients (6.4 percent) in whom the brain was the sole site of first recurrence. Sixty patients (3.9 percent) had only intracranial involvement at the time of death. Prognostic variables that had a significant effect on the time to recurrence in the brain were histologic features of the carcinoma (patients with nonsquamous-cell cancers were more at risk than those with squamous-cell cancer), the T1N1/T2N0 and T2N1 staging subsets (T1, tumor less than or equal to 3 cm in diameter; T2, tumor greater than 3 cm; N0, no regional lymph-node metastasis; N1, ipsilateral hilar-lymph-node metastasis), and initial weight loss of more than 10 percent. We conclude that prophylactic cranial irradiation would at best benefit only a very small subset of these patients. We believe, therefore, that neither prophylactic cranial irradiation nor a randomized trial is indicated in patients with non-small-cell lung cancer who have undergone complete resection.