Serum and peritoneal fluid collected from 48 patients at laparoscopy was assayed for CA 125. Geometric mean serum CA 125 was similar in 21 patients with predominantly early stage endometriosis (13.4 U/ml; confidence intervals (C.I.) 10.8-16.7) and 27 patients without endometriosis (12.9 U/ml; C.I. 9.7-17.2). Mean peritoneal fluid CA 125 was higher in the endometriosis patients (mean 44.4 U/ml; C.I. 34.5-57.2) than controls (35.7 U/ml; C.I. 30.0-42.2), but the difference was not significant. Peritoneal fluid volume was increased in endometriosis patients (p = 0.013), and only after correction for peritoneal fluid volume did total antigenic concentration in aspirated peritoneal fluid show significant elevation (p = 0.0015) in endometriosis patients (mean 340 U/ml; C.I. 212-547) compared with controls (125 U/ml; C.I. 85-184). These results suggest that although CA 125 may be expressed in peritoneal fluid in endometriosis, it is unlikely to be of use as a marker of disease activity in peritoneal fluid obtained by culdocentesis.