In a series of 641 patients with small cell lung cancer, 189 (29.5%) had at least one neurologic disorder either at the time of presentation or during the subsequent clinical course of the cancer. The total number of neurologic disorders was 210, which included brain metastases (75.7%), meningeal carcinomatosis (6.7%), intramedullary metastases (2.4%), epidural metastases (11.0%), hyponatremia producing CNS symptoms (3.3%), and Eaton-Lambert syndrome (1.0%). The most common signs and symptoms were motor dysfunction and confusion. The overall survival of patients with any neurologic disorder was compared to that of patients without neurologic problems. There was no difference between the survival curves for the first year and a half, but patients without neurologic complications had a greater probability of long-term survival (log-rank P = 0.03). There were no statistically significant differences when this comparison was made according to stage of disease. When a neurologic disorder related to cancer occurred, the survival time from the date of that diagnosis was usually short. The neurologic disorder was the immediate cause of death in the majority of cases. In patients who achieved a complete remission, the administration of prophylactic cranial irradiation (PCI) significantly reduced the risk of developing brain metastases as the initial site of the relapse (log-rank P = 0.0034). After adjustment for performance status and extent of disease, the survival of complete responders treated with and without PCI was not significantly different. We conclude that neurologic complications are a frequent and serious problem in patients with SCLC.