BACKGROUND: When hormones are detected in the serum of patients with bronchial carcinoma they are generally considered to originate from the tumour, but this may be not the only explanation. Pulmonary endocrine cells proliferate in lungs affected by non-neoplastic disease and their products are often demonstrable in the serum. The aim of this study was to examine the pulmonary endocrine systems of a series of tumour-bearing lungs to see whether any changes in them could possibly account for raised levels of pulmonary peptides in the blood. METHODS: The morphology, number, distribution, and content of pulmonary endocrine cells in 30 pairs of tumour-bearing lungs from patients coming to necropsy with bronchial carcinoma were examined. These features were related to the pathology of the tumour and to other pathological changes present in the lungs, and compared with pulmonary endocrine cells in 10 pairs of control lungs from patients without pulmonary disease. RESULTS: Increased numbers of endocrine cells, often in the form of large abnormal aggregates, were present in 17 pairs of tumour-bearing lungs where they were associated not with the tumour but with non-tumoral pathology, especially inflammation and changes associated with cardiac failure. Appropriate and inappropriate peptides were identified within them. CONCLUSION: The possibility is raised that, in some subjects with bronchial carcinoma who have raised serum hormone levels, the source of these substances might be the endocrine cells in the diseased lung around the tumour.