PURPOSE The purpose of our study was to evaluate the feasibility of magnetic resonance imaging (MRI) using ultrasmall superparamagnetic iron oxides (USPIO) in the detection of experimentally induced endometriosis. MATERIALS AND METHODS Endometriosis was surgically induced in rats by transplanting an autologous fragment of uterine tissue onto the inner surface of the abdominal wall, the posterior surface of the uterine body and the arterial cascades of the small intestines adjacent to mesenteric blood vessels. Six weeks later, MRI using Gd-DTPA and USPIO was performed for the evaluation of the ectopic uterine tissue (EUT). A scoring system was developed for image interpretation (0=absence, 1=probably absence, 2=probably presence and 3=presence). We defined MR index (MRIx) as the sum of T1-weighted and enhanced T1-weighted and T2-weighted image scores, and USPIO MRIx (MRIx(+USPIO)) as the MRIx score plus the score of USPIO-enhanced T2-weighted image. RESULTS The MRIx(+USPIO) was also higher in the successfully autotransplanted group than in the failed group (6.19±1.72 versus 3.94±1.20, PCONCLUSION Our results suggest that USPIO-enhanced MRI could be a novel diagnostic tool for diagnosis in experimentally induced peritoneal endometriosis.