CA-125 has been found to be mildly elevated in the serum of patients with advanced stages of endometriosis, but not with minimal endometriosis. Peritoneal fluid levels were measured in conjunction with serum levels in 12 consecutive patients with endometriosis and 12 consecutive normal controls. Apparent peritoneal fluid values were found to be 10-fold higher than serum levels, with no difference between the study groups (130 versus 106 U/ml, respectively). However, when peritoneal fluid was diluted to validate the assay system in peritoneal fluid, no sample of peritoneal fluid showed decreased binding with serial dilutions of 1:2, 1:5, and 1:10. At a dilution of 1:100, there was some decreased binding, but the level was 17-fold higher than expected. The source of this interference is unknown.