Expression of insulin-like growth factor-I (IGF-I), its receptor, and IGF- binding proteins (IGFBPs) by epithelial ovarian cancer cells and its mitogenic effect on these cells in vitro suggest that IGF-I may have a role in the regulation of human ovarian cancer. To examine this role in vivo, we explored the IGF-I/IGFBP system in sera and cyst fluids of women undergoing surgery for simple and other benign cysts (n = 20) and borderline (n = 5) and invasive malignant epithelial neoplasms (n = 11). The IGF-I level was significantly higher in cyst fluid from invasive malignant compared to cyst fluid in benign neoplasms (16.1 ± 2.2 vs. 7.3 ± 1.2 nmol/L; P = 0.002). Although benign cysts contained almost no IGFBP, high IGFBP-2 levels were detected in malignant cysts regardless of histological type. Serum IGFBP-2 levels were also higher in women with invasive malignancy than in benign controls (1.32 ± 0.32 vs. 0.53 ± 0.07 relative units; P = 0.004). IGFBP-2 was higher in cyst fluids than in the corresponding sera, implying local production of this protein. Estradiol was high in fluid from invasive malignant cysts of postmenopausal women and correlated with IGF-I in the cyst fluid. Levels of IGF-I, IGFBP-2, and estradiol in cyst fluid could discriminate between benign and malignant neoplasms, except for the endometrioid-type tumors (n = 2). Our data support a role for IGF-I in the proliferation of ovarian cancer and suggest that IGF-I and estradiol interact in regulating this malignancy.