Elevated levels of CA-125, an antigen expressed by malignant ovarian tissue, have been found in women who developed OHSS in response to treatment with exogenous gonadotropins for IVF. In contrast, CA-125 concentrations in women who were treated with an identical regimen but who did not show signs of OHSS remained in the normal range. We conclude that the expression of CA-125 is not restricted to neoplastic ovarian tissue but can also occur as a consequence of supraphysiologic stimulation of the ovary with exogenous gonadotropins.