Carcinoid tumors of the lung are uncommon and usually have a favorable outcome after resection. However, some patients have aggressive tumors that cannot be completely resected or have unfavorable outcomes after resection. A retrospective study was conducted to identify factors that predict outcome. Records of all previously untreated patients treated at the University of Texas M. D. Anderson Cancer Center (UTMDACC) between 1959 and 1994, with histologically confirmed carcinoid tumors of the lung, were reviewed. Histologic specimens were reclassified by contemporary criteria. Five-year survival for 79 patients without distant metastasis who had tumors with clinically aggressive features at diagnosis (defined as extension to the carina, involvement of regional lymph nodes, and/or vascular invasion) was 50.9% as compared with 76.9% (p = 0.01) for patients without these features. Five-year survival was 38.7% for patients with "atypical" histopathology (p = 0.01). Patients with aggressive tumors were more likely to have atypical tumors [odds ratio (OR) 11.73, p < 0.001]. However, 17.5% of the patients with clinically aggressive carcinoid tumors had tumors that were classified as typical. Carcinoid tumors of the lung that exhibit aggressive behavior may not have atypical histopathologic features. Clinical features that suggest aggressive behavior and unfavorable outcome warrant further investigation.