Graafian ovarian follicles consist of follicular fluid, one single mature oocyte, and several hundred thousands of granulosa cells (GCs). Until now, luteinizing GCs have been considered to be terminally differentiated, destined to undergo death after ovulation. Present concepts of luteal function, endocrine regulation of early pregnancy, and the recruitment of new ovarian follicles are all based on the cyclical renewal of the entire population of GCs. We now demonstrate that luteinizing GCs isolated from the ovarian follicles of infertile patients and sorted with flow cytometry based upon the presence of their specific marker, the follicle-stimulating hormone receptor (FSHR), can be maintained in culture over prolonged periods of time in the presence of the leukemia-inhibiting factor (LIF). Under those conditions the markers of GC function such as FSHR and aromatase gradually disappeared. POU5F1 (POU domain, class 5, homeobox 1), a typical stem cell marker, was expressed throughout the culture, but germ line cell markers such as nanog, vasa, and stellar were not. Mesenchymal lineage markers such as CD29, CD44, CD90, CD105, CD117, and CD166, but not CD73, were expressed by substantial subpopulations of GCs. The multipotency of a subset of GCs was established by in vitro differentiation into other cell types, otherwise not present within ovarian follicles, such as neurons, chondrocytes, and osteoblasts. Follicle-derived stem cells were also able to survive when transplanted into the backs of immunoincompetent mice, in vivo generating tissues of mesenchymal origin. The unexpected findings of multipotency of cells with prolonged lifespans originating from ovarian follicles are likely to have a significant impact on evolving theories in ovarian pathophysiology, particularly with reference to ovarian endometriosis and ovarian cancer.