In order to assess the usefulness of prophylactic cranial irradiation (PCI) in the treatment of small cell lung cancer, 43 complete responders to chemotherapy were randomized either to receive PCI or to have observation only between April 1981 and September 1986. Twenty-one patients were given 40 Gy of PCI in 20 fractions over 4 weeks after they had achieved complete response. Of those, 5 patients (24%) developed brain metastases within a median follow-up period 47.5 months (range, 15.5 to 67 months), while 11 of 22 patients (50%) not receiving PCI developed brain metastases within a median follow-up period of 40.5 months (range, 13 to 70 months). Cumulative actual probability of a brain metastasis was 5% at 12 months and 25% at 24 months for those receiving PCI, while it was 38% at 12 months and 53% at 24 months for those not receiving PCI. Thus, the probability of a brain metastasis was significantly lower in the PCI group than in the control group. The median survival time was 21 months for the PCI group and 14.7 months for the control group. The 2-year survival rate was also higher in the PCI group (35% versus 14%). However, the differences were not statistically significant due to the small number of patients. These interim results suggest the possibility of reducing brain metastases and prolonging survival by PCI in complete responders to chemotherapy.