Hepatocyte nuclear factor 4 alpha (HNF4 alpha) is a member of the nuclear receptor superfamily and is expressed in several endodermal tissues. The aim of the present study was to examine the expression of HNF4 alpha on ovarian epithelial tumors with immunocytochemistry and immunohistochemistry using mAbs recognizing P1 and P2 promoter-driven HNF4 alpha. Ovarian mucinous adenoma, mucinous tumors of borderline malignancy, and mucinous adenocarcinoma had positive nuclear staining for HNF4 alpha (41/45, 91%). One-third (34%) of mucinous tumors had P1-positive staining and most had P1/P2-positive staining (93%). MUC2- and MUC5AC-positive staining was observed in 34% and 95% of mucinous tumors, respectively. The histological subtype of these mucinous tumors was not correlated with HNF4 alpha expression. On cytology it was found that cancer cells in the ascites from ovarian mucinous adenocarcinomas were HNF4 alpha positive, but tumor cells in ascites from other types of ovarian carcinomas were negative for HNF4 alpha. Thus, HNF4 alpha is demonstrated to be a useful marker for histological and cytological diagnosis of ovarian mucinous tumors.