BACKGROUND The available data on effectiveness of aromatase inhibitors in treating pain symptoms related to endometriosis is limited. We compared the efficacy and tolerability of the aromatase inhibitor letrozole combined with norethisterone acetate versus norethisterone acetate alone in treating pain symptoms. METHODS This prospective, open-label, non-randomized trial included 82 women with pain symptoms caused by rectovaginal endometriosis. Patients received either a combination of letrozole and norethisterone acetate (group L) or norethisterone acetate alone (group N) for 6 months. Changes in pain symptoms during treatment and in the 12 months of follow-up were evaluated. Side effects of each treatment protocol were recorded. RESULTS Intensity of chronic pelvic pain and deep dyspareunia significantly decreased during treatment (P < 0.001 versus baseline by 3 months) in both study groups. At both 3- and 6-month assessment, the intensity of chronic pelvic pain (P < 0.001, P = 0.002, respectively) and deep dyspareunia (P < 0.001, P = 0.005, respectively) was significantly lower in group L than group N. At completion of treatment, 63.4% of women in group N were satisfied with treatment compared with 56.1% in group L (P = 0.49). Pain symptoms recurred after the completion of treatment; at 6-month follow-up no difference was observed in the intensity of pain symptoms between the groups. Adverse effects were more frequent in group L than in group N (P = 0.02). CONCLUSIONS The combination drug regimen was more effective in reducing pain and deep dyspareunia than norethisterone acetate; however, letrozole caused a higher incidence of adverse effects, cost more and did not improve patients' satisfaction or influence recurrence of pain.