In this study we examined intermediate filament expression in 45 formalin-fixed mesotheliomas. Immunostaining for cytokeratin was found in 86%, for vimentin in 71%, and for desmin in 4%; none stained for glial fibrillary acidic protein or neurofilament. The two biphasic mesotheliomas which expressed desmin also expressed smooth muscle actin but were negative for myoglobin. This, together with the ultrastructural findings, was taken as unequivocal evidence of a leiomyoid form of mesothelioma which might easily be confused with leiomyosarcoma. Both of these tumours co-expressed cytokeratin, exemplifying the value of cytokeratin immunostaining in the distinction between mesothelioma and sarcoma. Consistent non-expression of glial fibrillary acidic protein in mesotheliomas may help to distinguish them from nerve sheath tumours.