Amplification and overexpression of the cell cycle-related gene cyclin D1 have been demonstrated in several human malignancies and have been shown to be directly oncogenic in breast epithelium and lymphocytes. Overexpression of the gene can occur in the absence of gene amplification. We have investigated whether cyclin D1 is overexpressed in a panel of 43 sporadic epithelial ovarian cancers using immunohistochemistry. Cyclin D1 was overexpressed in 26% of these tumors. Overexpression of cyclin D1 is associated with borderline or well-differentiated, grade 1 tumors but does not correlate with a particular histological type, overexpression of the c-erb-B2 oncogene, or presence of estrogen receptors. It is suggested that overexpression of cyclin D1 may contribute to the pathogenesis of epithelial ovarian cancers, including a subset of tumors different from those overexpressing the c-erb-B2 oncogene.