Paraffin sections of 66 surgically resected lung tumours were immunostained with antisera to human endothelin-1 and to the C-terminal peptide of big endothelin. With both antisera, strong immunoreactivity was demonstrated in 11 of 15 squamous cell carcinomas and 11 of 16 adenocarcinomas. Focal immunoreactivity was seen in small cell carcinoma (2/12), large cell carcinoma (2/5), and carcinoid tumours (2/11). Four lymphomas and three sarcomas did not show endothelin immunoreactivity. Cryostat sections of 22 of the 66 tumours were hybridized with radiolabelled complementary RNA probes prepared from the 3' non-coding region of endothelin-1 cDNA, and the chromosomal genes encoding endothelin-2 and -3. In situ hybridization demonstrated the presence of endothelin mRNAs in 4 of 7 squamous cell carcinomas and in 5 of 8 adenocarcinomas, in a pattern similar to that shown by immunocytochemistry. No hybridization signals were obtained from the other types of tumours. In lung tissue adjacent to the tumours, endothelin-like immunoreactivity and mRNA were detected in pulmonary endocrine cells and, in some cases, other epithelial cells, and in alveolar capillary endothelial cells. This study demonstrates the expression of endothelin in a number of pulmonary tumours and suggests a possible role for this peptide in the growth and/or differentiation of these tumours.