Most epithelial ovarian cancers are diagnosed postmenopausally, although the well-established epidemiological risk factors (parity, oral contraceptive use) are premenopausal. We hypothesized that accumulation of senescent fibroblasts, together with concomitant loss of presenescent fibroblasts within the ovarian cortex, promotes initiation and early development of ovarian cancer from ovarian surface epithelial (OSE) cells. To test this, we established immortalized OSE (IOSE) cell lines that mimic early neoplastic transformation by overexpressing the CMYC oncogene (IOSE(CMYC)) and normal ovarian presenescent (PSN) and senescent (SEN) fibroblast cell lines. We then evaluated the ability of PSN and SEN fibroblasts to transform IOSE and IOSE(CMYC) after coculture. SEN fibroblasts significantly enhanced neoplastic development of IOSE(CMYC) cells; there was an up to 15-fold increase in migration of IOSE(CMYC) cells cocultured with SEN fibroblasts compared with PSN fibroblasts. Conditioned medium from SEN fibroblasts promoted anchorage-independent growth of IOSE(CMYC) cells. We studied fibroblast-epithelial cell interactions in heterotypic three-dimensional spheroid models. Dual immunohistochemical staining of spheroids for a proliferation marker (MIB-1) and cytokeratin-18 indicated that SEN fibroblasts induce approximately a five-fold increase in proliferation of IOSE(CMYC) cells relative to cocultures with PSN fibroblasts. SEN, but not PSN fibroblasts, also induced nuclear atypia in epithelial cells in three-dimensional spheroids. These data suggest for the first time that the accumulation of senescent, or loss of presenescent fibroblasts, can promote neoplastic development of partially transformed OSE cells in vitro and illustrates the power of using three-dimensional heterotypic modeling to gain better insights into the etiology underlying the development of epithelial ovarian cancer.