Desmin is a marker of smooth and striated muscle, but evidence is accumulating that it may be expressed by human mesothelium. The aim of this study was to describe desmin expression in normal, reactive, and hyperplastic peritoneal mesothelium, and to evaluate its potential use as a marker for differentiating between epithelial and mesothelial proliferations. We immunohistochemically studied 27 tissue specimens (from 22 patients) with reactive mesothelium, including omentum (n = 14), fallopian tubes (n = 7), ovaries (n = 3), ascitic fluid (n = 1), and peritoneal washings (n = 2). Ovarian surface epithelium (OSE) from 9 cases and 28 ovarian surface epithelial tumors was evaluated for comparison. The desmin expression pattern in the mesothelium, which was similar to but less consistent than that of cytokeratins, was evident in flat and reactive mesothelium, including hyperplastic mesothelial sheets and mesothelium entrapped in clefts. Mesothelial pseudoglandular structures, present in three cases, were predominantly negative for desmin. Desmin expression was observed in the OSE in 4 of 9 cases but not in any mullerian-derived epithelium or mullerian type tumor. Thus, in contrast to cytokeratins, desmin discriminated mesothelial cells from mullerian type epithelia. Compared with vimentin, desmin discriminated mesothelial cells from other tissues except muscle cells. We conclude that desmin may be used in addition to cytokeratins and vimentin as a marker of peritoneal mesothelium.