Carcinoid tumors of the lung in 10 patients were treated surgically and both the clinicopathological manifestations and immunohistochemistry were examined in detail. Five were central carcinoid tumors, located in the main, lobar or segmental bronchus and five were peripheral carcinoid tumors, located in the subsegmental bronchus or beyond. Histologically, eight of the tumors were typical carcinoid tumors, one was an atypical carcinoid tumor, and one a carcinoid tumorlet. Three growth types were also established: polypoid type, iceberg type and intrapulmonary type. The central carcinoid tumors belonged either to the polypoid type or iceberg type, while the peripheral carcinoid tumors were of the intrapulmonary type. Both the iceberg and intrapulmonary types may invade the peribronchial or parenchymal tissues more frequently than does the polypoid type. Immunohistochemically, argyrophilia and neuron-specific enolase (NSE) were detected in all the tumors examined and six stained for polypeptide hormones such as adrenocorticotropic hormone (ACTH) and/or pancreatic polypeptide (PP). Of these, five had epithelial markers such as keratin, epithelial membrane antigen (EMA) and/or carcino-embryonic antigen (CEA). These findings suggest that a carcinoid tumor of the lung originates from primitive multipotential stem cells such as those of a neuroendocrine or epithelial nature.