A consecutive series of 38 lung carcinoid tumours (36 surgical and two necropsy specimens) was studied. Histopathological features and amine and peptide hormone immunoreactivity were correlated with gross characteristics (size, location) and clinical data. Peripheral carcinoids were detected a decade later than central carcinoids and tended to be bigger. In general, the histological characteristics of peripheral and central carcinoids were similar; atypical features, however, were more common in peripheral carcinoids. Most carcinoids contained many argyrophilic cells (58%). Although argentaffinic cells were not found, serotonin immunoreactive cells were present in 32% of the tumours. Peptide hormone immunoreactivity (adrenocorticotrophic hormone (ACTH), calcitonin, somatostatin, gastrin) was rare. In one case massive ACTH production had caused clinically manifest Cushing's syndrome. In two other cases few ACTH immunoreactive cells were found and in one case calcitonin immunoreactive cells were present. The relative rarity of hormone production in lung carcinoids and the predominantly benign course of the tumour preclude the use of peptide hormone production as a prognostic indicator.