In an attempt to define the problem of brain metastases in patients with small cell lung cancer (SCLC), I analysed a series of 82 patients between 1972 and 1981. Brain metastases were documented in 10 of the 82 patients (12%) at diagnosis, and in 19 of the remaining 72 patients (26%) after onset of chemotherapy. The incidence of brain metastases developing after onset of chemotherapy was 24% (10/42) for those with limited disease, and 30% (9/30)for those with extensive disease. As for the response to chemotherapy, brain metastases were much more frequent in complete responders than in partial or non-responders; 44% for complete responders, 16% for partial responders, and 19% for non-responders. Two of the 25 complete responders relapsed in the brain 9 and 42 months, respectively, after beginning chemotherapy. They died of uncontrollable neurologic disease despite of a temporary response to brain irradiation. The probability of developing a brain metastasis increased with lengthening of patient survival to 60% after 3 years. Systemic chemotheapeutic agents with cross the blood-brain barrier appeared not to prevent the frequency of brain metastases. Althogh therapeutic irradiation for brain metastases provided adequate palliation and the majority died of systemic disease rather than neurologic disease even in complete responders, prophylactic therapy appears necessary for those who achive a complete response in order to cure SCLC.