Considerable attention has been devoted to the diagnosis of small cell lung carcinoma (SCLC) and its subtypes. In the literature contradictory opinions have been published concerning the clinical implications of subtyping, largely because of the different criteria used by different pathologists. This article is a consensus report by the Pathology Committee of the International Association for the Study of Lung Cancer. The following classification of SCLC is recommended: (1) Small cell carcinoma. This subtype includes most of the tumors previously included in the oat cell and intermediate subtypes. More than 90% of untreated SCLC fall into this category. (2) Mixed small cell/large cell carcinoma. This subtype, which may be associated with a poor prognosis and response to therapy, contains a spectrum of cell types ranging from typical SCLC to larger cells having prominent nucleoli and resembling large cell carcinoma. (3) Combined small cell carcinomas. Typical SCLC elements are intimately admixed with areas of differentiated squamous cell or adenocarcinoma. This simplified classification of SCLC will facilitate uniformity in the diagnosis and further our understanding of the clinical significance of the rarer SCLC with variant morphologies.