The object of this study was to describe the histology and immunohistochemistry of 13 small cell mesotheliomas, concentrating on reliable distinctions between them and small cell carcinoma. All 13 tumours showed regions of more typical mesothelioma if multiple blocks were examined. No tumours showed the streams, ribbons, rosettes, or haematoxyphilic blood vessels that are typical of small cell carcinoma. Mitotic figures were relatively scarce and the nuclei had a particularly characteristic open appearance with prominent nucleoli and delicate chromatin. Nuclear moulding was not seen. No tumour produced neutral mucin. Immunohistochemical positivity for neuron-specific enolase (NSE) was found in 11/13, cytokeratin in 9/13 and Leu-7 in 4/13 but none was positive for chromogranin A, carcino-embryonic antigen (CEA) or leucocyte common antigen (LCA). We conclude that the accurate diagnosis of small cell mesothelioma is possible, provided that the clinical presentation is known, the tumour is adequately sampled and the microscopy carefully assessed. In small biopsy specimens, where the diagnosis is less straightforward, immunohistochemistry for CEA, and perhaps LCA and chromogranin A may be helpful. NSE and Leu-7 positivity is common in these tumours and might be misleading if interpreted as reliable evidence of neuroendocrine differentiation.