Objective: To evaluate the efficacy and safety of an intrauterine system releasing 20 mu g of levonorgestrel per 24 hours in the long-term treatment of recurrent dysmenorrhea in women already operated on conservatively for endometriosis. Design: A prospective noncomparative pilot study. Setting: A tertiary care and referral academic center for patients with endometriosis. Patient(s): Twenty parous women with recurrent moderate or severe dysmenorrhea after conservative surgery for endometriosis who did not want further children. Intervention(s): A levonorgestrel-releasing intrauterine system was inserted in each woman within 7 days of the start of a menstrual cycle. Main Outcome Measure(s): Variations in severity of dysmenorrhea during treatment according to a 100-mm visual analogue scale and a 0-3-point verbal rating scale, modification of a pictorial blood-loss assessment chart devised to evaluate the amount of menstrual flow, and degree of satisfaction after 12 months of therapy. Result(s): One woman was lost to follow-up after achieving amenorrhea and expressing satisfaction, and 1 requested system removal because of weight gain and abdominal bloating. In another subject, the levonorgestrel intrauterine system was expelled 3 months after insertion. The menstrual patterns in the remaining 17 women were characterized by amenorrhea in 4 cases, hypomenorrhea or spotting in 8, and normal flow in 5. Baseline and 12-month follow-up mean +/- Sn blood loss scores were 111 +/- 36 and 27 +/- 26, respectively. At the same time, mean +/- SD visual analogue and verbal rating scale scores dropped, respectively, from 76 +/- 12 to 34 +/- 23 points and from 2.5 +/- 0.5 to 1.2 +/- 0.5 points. Four women were very satisfied with treatment, 11 were satisfied, 2 were uncertain, and 3 were dissatisfied at 12-month follow-up. Conclusion(s): Because of the amenorrhea or hypomenorrhea induced in most women, a levonorgestrel intrauterine system greatly reduced menstrual pain associated with endometriosis and achieved a high degree of patient satisfaction. (Fertil Steril(R) 1999;72:505-8. (C) 1999 by American Society for Reproductive Medicine.).