AIMS--To compare in situ end-labelling (ISEL) of apoptosis in lung carcinoma with quantitative and semiquantitative light microscopic assessment and ultrastructural observations. METHODS--ISEL of apoptosis was evaluated in 42 lung carcinomas (24 squamous cell carcinomas, 12 adenocarcinomas and six small cell carcinomas). Results were correlated semiquantitatively with the extent of apoptosis in haematoxylin and eosin stained sections, with apoptotic indices and with ultrastructural observations (nine cases). RESULTS--In each tumour type the extent of apoptosis identified by ISEL correlated with that observed on light and electron microscopy. Tumour cells undergoing apoptosis showed either uniform nuclear staining with a surrounding "halo" or peripheral nuclear membrane staining. The latter pattern was more prominent in small cell carcinoma and correlated ultrastructurally with early apoptosis. A variable proportion of apoptotic cells and apoptotic bodies were unlabelled. Necrotic tumour cells were weakly stained but were distinguishable from apoptotic cells. CONCLUSIONS--ISEL, if used in conjunction with standard methods for investigating apoptosis, is a useful adjunct to the investigation of apoptosis in human tumour tissue.