Relaxin-like factor (RLF), a new member of the insulin-like growth factor family, is a reliable marker for normal Leydig cells in the human postpubertal testis (1). Expression of the RLF gene appears to be developmentally regulated, given that only during puberty is RLF expression up-regulated. We recently demonstrated down-regulation of the human RLF gene in testicular Leydig cell tumors indicating dedifferentiation of the Leydig cells within the tumor (2). Ovarian Sertoli-Leydig cell tumors (SLCTs), histologically typed as androblastomas, are rare, potentially malignant sex-cord stromal tumors exhibiting testicular-like structure and differentiation of various degrees. In the present study, we investigated the expression of RLF, 17alpha-hydroxylase, 3beta-hydroxysteroid dehydrogenase (3beta-HSD), Ki-67, and cytokeratin 18 in a human ovarian SLCT of low differentiation.