Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The quality of cervical mucus (CM) among the levonorgestrel-releasing intrauterine system (LNG-IUS) users is controversial. The objectives were to assess CM compared to the levels of oestradiol (E-2) and the frequency of cycles with luteal activity among users of the LNG-IUS.Materials and methods: In total, 224 LNG-IUS users for between two months and five years were recruited at a Brazilian family planning clinic. For the cross-sectional part of the study, we enrolled 175 LNG-IUS users at 2, 6 12, 24, 36, 48, and 60 months after insertion (25 women in each group), and we performed one evaluation. For the prospective part of the study, we enrolled 49 LNG-IUS users at the same lengths of use after insertion (7 women in each group), and we evaluated these women once a week for five consecutive weeks. (.)Results: Mean ( SEM) CM scores of all evaluations among women with single and weekly evaluations were between 3.30.9 and 8.5 +/- 0.3, respectively independently of the length of use of the LNG-IUS. Mean E-2 values ranged from 45.5 +/- 6.8 to 472.5 +/- 34.7pg/ml and the maximum ovarian follicle diameter on the days of evaluation varied from 14.0 +/- 1.3 to 31.2 +/- 0.4mm.Conclusions: The mean CM score of all evaluations, independent of the length of use of the LNG-IUS and normal levels of serum E-2, was below 10 was according to the WHO is inadequate for sperm penetration.214318322National Research Council (CNPq) [573747/2008-3]International Contraceptive Access (ICA) Foundation, Turku, FinlandConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)