The levels of the oncofetal antigens CA-125, CA 19-9 and CA 15-3 were measured in serum samples taken from 8 women, aged 21 to 37 yr, before treatment, during the last fifteen days of a 6-month administration of danazol and finally three months after treatment withdrawal. The purpose of the study was to investigate: i) whether endometriosis belongs to the pathologic conditions which induce a concomitant increase in the values of CA-125, CA 19-9 and CA 15-3, and ii) the effect of danazol on the levels of these antigens. Our results indicate that before danazol treatment, three women showed pathologic levels of all three antigens, one of CA 19-9 and CA 15-3, one of CA 19-9 alone, and two of CA 15-3 alone. Administration of the drug significantly reduced the levels of CA-125 (p less than 0.001) and CA 19-9 (p less than 0.05) and to a lesser degree the levels of CA 15-3. Three months after danazol treatment discontinuation, the levels of these three antigens remained significantly lower (p less than 0.05) than the respective pretreatment values. Our findings substantiate the view that endometriosis must be classified with the pathologic conditions which induce a rise in the levels of all three antigens, and that ovarian function mainly influences the levels of CA-125 and CA 19-9.