To investigate whether the aminoterminal propeptides of type III procollagen are increased in patients with early endometriosis and to demonstrate that the subtle lesion of endometriosis is an active stage of the disease. Aminoterminal propeptide of type III procollagen was determined in serum and peritoneal fluid (PF) of 100 consecutive patients undergoing laparoscopy. Academisch Ziekenhuis Maastricht, The Netherlands, a tertiary care center. Aminoterminal propeptide PF levels were significantly higher in women with early lesions of endometriosis compared with levels in two groups of controls, i.e., fertile, cycling, women without the disease (P=0.019) and women on oral contraceptives without the disease (P=0.036). No difference was found in aminoterminal propeptide PF levels when comparing patients with early lesions of endometriosis and patients with unexplained infertility, the third control group. Aminoterminal propeptide PF levels of patients with endometriosis without early lesions were not different from PF levels in controls. The early lesion is an active stage of endometriosis, invading the extracellular matrix. In women with unexplained infertility active, microscopic endometriosis may be present.