Oct-4 (Octamer-binding transcription factor 4) is a member of the POU (Pit-Oct-Unc) family. During development, Oct-4 is expressed in embryonic stem cells and in germ cell precursors. In this study, we investigated the expression of Oct-4 in the ovaries of human fetuses during gestation. The ovaries of 14 human fetuses and newborns, ranging in gestational age from 12 up to 38 weeks of gestation, were formalin-fixed, routinely processed and paraffin-embedded. Paraffin sections were immunostained with an anti-Oct-4 commercial antibody. Oct-4 expression was demonstrated in all the ovaries analyzed. Immunoreactivity for Oct-4 was detected in multiple stem/progenitor cells, including oogonia. Moreover, Oct-4 was expressed in oocytes, in primordial follicles. In ovarian stem/progenitor cells, Oct-4 was expressed in the nucleus, whereas in oocytes reactivity for Oct-4 was restricted to the cytoplasm. In the initial stages of gestation, the majority of Oct-4-positive precursor cells were detected in the external cortex. These preliminary data indicate Oct-4 as a major player in germ cell differentiation in the human ovary and as a useful marker for ovarian stem/progenitor cells. Given the ability of Oct-4 for the detection of ovarian stem/progenitor cells, further studies are needed in order to verify its ability to detect stem cells in adult ovaries.