Background: The objective of this study was to evaluate the efficacy and side-effects of the levonorgestrel-releasing intrauterine system (LNG-IUS) in the treatment of moderate or severe dysmenorrhea associated with adenomyosis and the influence on ovarian function. Study Design: The LNG-IUS was inserted into 60 women who had moderate or severe dysmenorrhea associated with adenomyosis diagnosed by transvaginal sonography. A visual analogue scale (VAS) of dysmenorrhea, uterine volume and serum-levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), etstradiol (E2), and prolactin (PRL) were used to assess the efficacy of the treatment at baseline and at six and 12 months after the LNG-IUS. Serum-levels of FSH, LH, E2, and PRL were tested in pre-and post-insertion at six and at 12 months, respectively. Side-effects were recorded at every follow-up visit. Results: After six and 12 months of LNG-IUS insertion, dysmenorrhea was obviously alleviated, and the dysmenorrhea scores decreased to 2.6 from 0.6 (p < 0.05). The volume of uterus reduced six months after insertion and later, but without significant change (p < 0.05). After treatment of serum, in terms of FSH, LH, and E2 levels, compared with pre-insertion, there was no statistically significant difference (p > 0.01). However, the level of PRL markedly declined at six and 12 months after LNG-IUS. Conclusion: The LNG-IUS appears to be an effective method in alleviating dysmenorrhea associated with adenomyosis with little effect on ovarian function. It may be helpful to decrease the level of PRL in these patients.