We measured serum levels of CA 125 in 26 patients with external endometriosis, using CA 125 RIA Kits. The normal range was instituted below 39 U/ml. The mean CA 125 level (+/- SD) was 55.1 +/- 24.9 U/ml in patients with external endometriosis, the positive rate being 71.4%. Considering the clinical stage of external endometriosis, the mean CA 125 level and positive rate increased to 64.5 +/- 23.2 U/ml and 93.3%, respectively. The CA 125 levels in these patients gradually decreased after surgery and/or Danazol treatment and the levels were below 39 U/ml within four weeks. Thus, progressive external endometriosis can be accurately diagnosed by determining the serum levels of CA 125, and appropriate clinical treatment designed.