Abnormally expressed oncogenes are implicated in neoplastic transformation. We have investigated a series of endocrine tumours using immunocytochemistry as a first screening tool to detect oncogene expression. Paraffin sections of 44 pulmonary small cell carcinomas, 15 pulmonary atypical carcinoids, 12 bronchial carcinoids, 28 medullary thyroid carcinomas, 27 phaeochromocytomas, and 17 insulinomas were immunostained with antibodies to c-erbB-2, c-myc, L-myc, and N-myc. Diffuse immunoreactivity was detectable for c-erbB-2 in 8 out of 44 (18 per cent) pulmonary small cell carcinomas, 3 out of 15 (20 per cent) pulmonary atypical carcinoids, and 6 out of 27 (22 per cent) phaeochromocytomas; for c-myc in 18 out of 44 (41 per cent) pulmonary small cell carcinomas and 5 out of 15 (33 per cent) pulmonary atypical carcinoids; for N-myc in 6 out of 28 (21 per cent) medullary thyroid carcinomas; and for L-myc in 4 out of 27 (15 per cent) phaeochromocytomas. There was considerable intratumoral and intertumoral heterogeneity and, in each tumour group, no relationship was found between tumour pattern, mitotic index, and oncoprotein immunoreactivity. These results suggest that oncogene products are present in a proportion of endocrine tumours, and that specific oncoproteins seem to be related to tumour type but not to other histopathological findings. Thus, oncoprotein detection may be a useful tool for identifying subsets of endocrine tumours that are not otherwise recognizable morphologically.