Serum CA 125 was measured with immunoradiometric assay in fourty-one patients of endometriosis Serum CA 125 levels (Mean ${pm}$ SEM, U/ml)in patients with endometriosis were 13.8${pm}$3.3 in stage I , 17.3${pm}$3.0 in stage II, 23.4${pm}$4.9 in stage III, and 64.5${pm}$13.2 in stage IV, which showed increment according to the stage of endometriosis. Serum levels in stage IV only were significantly elvated than those in control (15.3${pm}$2.2), and those in advanced stage(III and IV ; 47.1${pm}$9.5)were significantly higher than those in earlier stages (15.9${pm}$2.2)and control. The assay revealed a sensitivity of 31.7% and the frequency of elevated levels (> 30.3 U/ml)in stage I, II, III and IV was 9. 1%, 18.8%,33.3%, and 87.5% respectively. Sensitivity in advanced stage was higher than that in earlier stage (64.3% vs. 14.8%). There data suggest that serum CA 125 assay might be a useful diagnostic tool in the advanced stage of endometriosis.