Metallothioneins (MTs) are a group of low-molecular-weight proteins that are overexpressed in a variety of human neoplasms and are related to differentiation and prognosis in some tumor types. This study investigated immunohistochemically detectable metallothionein expression in benign and malignant ovarian surface epithelial tumors of serous, mucinous, and endometrioid types. MT expression was observed in 56% of carcinomas (n = 139) and in 2% of benign neoplasms (n = 81). Of the malignant tumors, MT expression was found in 68% of endometrioid, 56% of mucinous, and 52% of serous neoplasms. There was increased MT expression in grade 3 carcinomas (64%) as compared with grade 2 (60%) and grade 1 (23%). The overexpression of MT in malignant as opposed to benign ovarian surface epithelial tumors may suggest a role in tumorigenesis. Analogous to the situation in endometrial carcinomas, there is a tendency toward higher expression in poorly differentiated tumors. Whether high MT expression is an independent prognostic factor and increased expression indicates chemotherapy resistance in ovarian cancer, as has been previously suggested, should be determined by further studies.