Use of levonorgestrel-releasing intrauterine systems (LNG-IUSs) has been increasing. LNG-IUS use constitutes the preferred hormonal contraception among Danish premenopausal women older than 30 years. However, whether use of an LNG-IUS is associated with breast cancer risk remains unclear. Increased breast cancer risk was found in users of LNG-IUSs, but that study was not designed to address duration of continuous use. Also, previous studies have not adequately accounted for other hormonal contraceptive use. This study assessed breast cancer risk with continuous use of an LNG-IUS, accounting for other hormonal exposures.