The purposes of this study were to evaluate the effects of insulin-like growth factor (IGF)s in peritoneal fluid (PF) from patients with and without endometriosis on the proliferation of endometrial stromal cells and to investigate the effects of type I IGF receptor antibody on the response of endometrial stromal cells to PF from patients with endometriosis. IGFs in PF from patients with endometriosis (n=14) and without endometriosis (n=10) were measured by immunoradiometric assay and PF samples were divided into low IGF-I PF group (less than 85 ng/ml) and high IGF-I PF group (more than 85 ng/ml). Endometrial stromal cells from patients without endometriosis were cultured in serum free media in the presence or absence of 1 % PF and thymidine incorporation test were used to evaluate the proliferation of endometrial stromal cells. Also cultures were incubated with type I IGF receptor monoclonal antibody (${alpha}IR_3$) before adding PF. PF from patients with endometriosis and without endometriosis increased thymidine incorporation in endometrial stromal cells. In patients with endometriosis, high IGF-I PF group had high IGF-II levels and resulted in higher thymidine incorporation than low IGF-I PF group but no significant difference in increase in thymidine incorporation between high IGF-I and low IGF-I PF group was noted in patients without endometriosis. There was not a significant correlation between increase in thymidine incorporation and IGF-I levels in PF from patients without endometriosis but in PF from patients with endometriosis. Preincubation with ${alpha}IR_3$ significantly inhibited the mitogenic response of endometrial stromal cells to PF. Our data indicate that IGF-I in PF may be involved in the growth of ectopic endometrium in patients with endometriosis.