This study was performed to evaluate CA 125 in peritoneal fluid as an indicator of endometriosis. Peritoneal fluid from patients with mostly minimal and mild endometriosis (n = 43) and normal controls (n = 17) was collected at laparoscopy or laparotomy. The median concentration of CA 125 in peritoneal fluid did not differ significantly between patients and controls (79 IU/ml versus 76 IU/ml). In patients with endometriosis, a significantly increasing concentration of CA 125 in peritoneal fluid was seen from the early follicular to the late luteal phase; a similar change was not observed in the controls. In 14 patients, peritoneal fluid was sampled again after treatment with danazol and a significant reduction in median CA 125 concentration (76.5 IU/ml versus 57 IU/ml), peritoneal fluid volume (17.5 ml versus 10.5 ml) as well as reduced endometriosis scores (4 versus 2) were found. In controls, the concentration of CA 125 was about 10 times higher in peritoneal fluid than in serum. As the peritoneal levels of CA 125 did not differ significantly between patients with endometriosis and controls and as the reduction seen after danazol treatment did not correlate with the decrease of endometriotic implants, it is concluded that the monitoring of CA 125 in peritoneal fluid will not be useful in the diagnosis or control of endometriosis.