The levels of CA 125 in the serum was evaluated in 66 patients with endometriosis diagnosed and staged according to the revised American Fertility Society (AFS) classification via laparoscopy. The patients received a 6 month course of gonadotropin-releasing hormone (GnRH) agonist. Serum CA 125 levels were measured before, during (3 months and 6 months after the initiation of therapy) and 6 months after cessation of the medication. Patients with minimal and mild endometriosis had mean pre-treatment values significantly higher than control subjects in the luteal phase of the cycle or postmenopausal women (p<0.05) but the overall mean value was still below 35 U/ml. In contrast 80.7% of patients with moderate or severe endometriosis had levels in excess of 35 U/ml and the mean values for these groups were significantly elevated (p<0.005). Levels of CA 125 fell to those found in normal controls, during treatment, but rose again following cessation of the treatment. Nine of 19 subjects whose follow-up values of CA 125 exceeded 35 U/ml had a proven recurrence of endometriosis, while only 3 of 47 patients with values less than 35 U/ml had laparoscopically proven persistance or recurrence. The sensitivity and spesificity of CA 125 were 75% and 83.3% respectively and positive predictive value (PPV) was 46.36% as a predictor value of the recurrence. The data suggest that CA 125 levels may be a reliable indicator for monitoring the efficacy of GnRH agonist treatment of endometriosis, but its predictor value of recurrence is low.