To determine how endometrium alone would contribute to maintaining the circulating levels of Insulin-like growth factor binding protein-1 in vivo, serum immunoreactive IGFBP-1 levels were measured in 19 patients undergoing endometrial ablation using gynecologic resectoscopy. After endometrial ablation there was a significant decrease in the mean levels of circulating IGFBP-1, which was not correlated with the menstrual cycle. This result indicates that the endometrium is one of the sources of the circulating IGFBP-1.