OBJECTIVE To determine the long-term recurrence rate of endometriosis after treatment with gonadotropin-releasing hormone analogues (GnRH-a). METHODS A historical prospective study. METHODS Royal Free Hospital, London, a tertiary referral center for the treatment of endometriosis. METHODS One hundred thirty patients with endometriosis had treatment with GnRH-a buserelin acetate, goserelin, and nafarelin acetate between the years 1985 and 1987. Patients no longer being followed in the gynecology clinic were sent a questionnaire to determine their state of health. Information was also requested from the patient's general practitioner. METHODS The cumulative recurrence rate for the fifth year after treatment ended was 53.4%. RESULTS Patients with a higher disease stage at the outset were more likely to experience recurrence and experience it earlier than patients with minimal disease. Fifth-year recurrence rates were 36.9% for minimal disease and 74.4% for severe disease. The change in endometriosis stage classification scores at second-look laparoscopy for those patients whose disease recurred after treatment was not significantly different from those whose disease did not recur during the study period. CONCLUSIONS Patients with endometriosis treated with GnRH-a are highly likely to suffer a recurrence of their disease, particularly if their disease is severe at the outset.