A large proportion of patients with small cell lung cancer develop intracranial metastases which are often severely disabling. The optimal radiotherapeutic program for treating these metastases is unknown. We therefore evaluated objective response rates, response duration, and survival after therapeutic cranial irradiation in 59 patients with proven brain metastases from small cell lung cancer. Objective responses to a variety of doses and schedules were observed in 37 (63%) patients. However, progression of intracranial disease after radiotherapy was common, with 24 responding patients having relapsed in the brain prior to death. The actuarial likelihood of remaining free of progressive brain tumor at 1 year was only 37% in complete and 0% in partial responders. Patients who received radiation doses of more than 40 Gy had longer response durations than those given lower doses, although patient selection could well explain this observation. Brain metastases presenting after initiation of systemic chemotherapy or occurring in conjunction with other sites of extrathoracic disease were associated with a poor prognosis. In patients who present with brain metastases as the sole site of metastatic disease, higher doses of cranial irradiation should be considered, in view of the high intracranial relapse rate associated with currently accepted dose and fractionation schedules.