Seventy-one women underwent follow-up laparoscopy for persistent infertility six to 24 months after conservative treatment of endometriosis. The disease was staged according to the revised American Fertility Society classification of 1985. The serum CA 125 concentration was also measured in each patient to evaluate its efficacy in the diagnosis of endometriosis recurrence. The patients with stages I and II endometriosis had serum CA 125 levels not significantly higher than in the patients with negative findings, whereas those with stages HI and IV endometriosis presented significantly higher levels (P<.005 and P<.001, respectively) than the disease-free women. The sensitivity of serum CA 125 measurements in the diagnosis of endometriosis recurrence was 14.8%, the specificity was 100%, and the predictive values of normal (less than 35 U/mL) and elevated levels were 27 and 100%, respectively. In some cases, serum CA 125 measurements may be used instead of follow-up laparoscopy, or to indicate when laparoscopy should be performed or postponed.