The initial sites and frequencies of disease progression in 97 patients with small cell carcinoma of the lung treated in a Northern California Oncology protocol were analyzed. Among the extensive disease complete responders (25 patients), the chest was the most frequent initial relapse site (18 patients), followed by the liver (nine patients) and bone (six patients). For those patients who had a partial or no response to treatment, the chest was the most frequent site of persistent disease and the majority progressed in the chest initially. The addition of chest irradiation (5000 rad/5 weeks) to patients with limited disease significantly reduced the incidence of relapse (25%) and prolonged the disease-free interval in the chest in the complete responders, but did not affect the failure pattern in partial and nonresponders. All patients received prophylactic cranial irradiation and three limited disease patients (10%) and three extensive disease patients (4%) progressed in the brain.