Twenty-nine patients with small cell carcinoma of the lung and without evidence of brain metastasis were randomized into two treatment groups consisting of 14 patients who received prophylactic cranial irradiation (PCI) and 15 who received none (non-PCI). All patients were treated with irradiation of the primary lesion and concomitant chemotherapy. The response rate and median survival of the two groups were not significantly different: 93% and 7.2 months in the non-PCI; 86% and 9.8 months in the PCI; P larger than or equal to .05. Brain metastasis occurred in 0/14 patients in the PCI and 4/15 in the non-PCI (P less than or equal to .05) and was the cause of major neurologic disability in each. Although PCI did not improve response rate or survival, brain metastasis with its attendant neurologic complications was effectively prevented.