CA 125 levels were measured in the serum of 18 patients with laparoscopically diagnosed stage 2 to stage 4 endometriosis (American Fertility Society classification) and in eight normally cycling control women. All endometriosis patients were treated medically with either Danazol or Buserelin. CA 125 levels were measured during treatment and during the 18-month follow-up period. The mean CA 125 level in women with endometriosis was significantly higher than that observed in normal women (28.6 +/- 5.1 (+/- SE) units/mL vs. 11.1 +/- 1.1 units/mL). However, there was considerable overlap of values between controls and patients with stage 2 disease. The levels in patients with stage 3 or stage 4 disease were always greater than 20 units/mL. There was a significant positive correlation (r = .51) between the implant score and CA 125 levels, while there was no correlation between the total score (which includes adhesions and implants) and the CA 125 levels. Four of the patients who had recurrence of symptoms approximately 1 year after treatment had CA 125 levels close to pretreatment levels, and recurrence of endometriosis was confirmed by laparoscopy. The CA 125 levels in the rest of the patients remained suppressed during the follow-up periods. These results indicate that (1) CA 125 level can predict active endometriosis lesions in patients with stage 3 and stage 4 endometriosis, but is of no value for predicting adhesions; (2) CA 125 levels are useful in monitoring therapy during treatment; (3) during the follow-up period, elevations in CA 125 might predict recurrence of disease in women with stage 3 and stage 4 endometriosis.(ABSTRACT TRUNCATED AT 250 WORDS)