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Mesothelin is a cell surface glycoprotein that is present on normal mesothelial cells and overexpressed in several cancers. On immunohistochemical examination of a limited number of ovarian tumors, increased mesothelin expression has been previously noted. The authors evaluated mesothelin expression in 48 patients with ovarian cancer who were screened for participation in phase 1 studies of a recombinant immunotoxin targeting mesothelin. Eligibility criteria for participation in the studies included mesothelin expression by more than 30% of accessible tumor cells. Sections of formalin-fixed paraffin-embedded tumor specimens were evaluated for mesothelin expression by immunohistochemistry using the anti-mesothelin monoclonal antibody K1. Between September 2000 and January 2003, 48 ovarian tumors were analyzed for mesothelin positivity. Mesothelin positivity was noted in 34 of the 48 cases evaluated (71%). These results show that mesothelin is expressed in most epithelial ovarian cancers and that mesothelin expression in ovarian cancers can be evaluated in archival material. Patients whose tumors express mesothelin could be eligible for participation in clinical trials of novel agents targeting mesothelin.