We describe 17 atypical carcinoid tumors of the lung that are histologically intermediate between small-cell undifferentiated carcinoma and typical carcinoid tumor. Atypical carcinoid tumors have a distinctive microscopic pattern of nests, trabeculae, and ribbons of intermediate-sized, moderately pleomorphic cells that lack the nuclear molding and dense hyperchromasia of small-cell undifferentiated carcinoma. Mucin was present in 15 cases and nine contained argyrophilic granules. The three primary tumors and one metastasis studied ultrastructurally contained dense-core granules and variable degrees of squamous and glandular differentiation. Eleven patients were men; six were women. Their mean age was 58 years. Sixteen patients were known smokers. Twelve tumors involved the upper lobes and three were located in the right middle lobe. Mean tumor size was 4.9 cm (median 4; range 2.5--20). Four of 13 patients having potentially curative resection died of tumor. The other nine patients were disease free with a mean follow-up of 20 months. Four additional patients treated palliatively died.