The pattern of failure was studied in 1012 patients with resected Stage I or II non-small-cell carcinoma of the lung. Initial intrathoracic failure (41%) was more common than initial extrathoracic failure (34%) even though a complete resection was the intent in all patients. The most frequent sites of initial failure were the bronchial resection line (16%) and the central nervous system (CNS) (15%). The site distribution of initial failure does not appear to depend on TNM stage or pattern of nodal involvement. Patients with poorly differentiated disease had a greater rate of initial extrathoracic failure (p less than 0.01), predominantly bone or CNS. Implications for therapy and future research are discussed.