The term “isthmocele” refers to a niche on the anterior wall of the uterine isthmus or of the cervical canal at the site of a previous cesarean delivery scar. Such anatomic defect can cause many gynecologic sequelae that only recently have being identified and described. Hysteroscopy is commonly considered the gold standard for the diagnosis and also for the treatment, at least in the case of defects of small size. The authors described the case of a 37-year-old woman who underwent a cesarean section (CS) seven years before, with a long lasting history of menstrual irregularities, and pelvic pain increasing during menstruation at the hypogastric level. Magnetic resonance imaging (MRI) showed an exceptionally large isthmocele on the anterior wall of a retroflexed uterus which was otherwise misinterpreted as the uterine cavity filled with menstrual blood during a previous hysteroscopy (HSC). Although exceptional, this case highlights the possibility that a large sized isthmocele in a retroflexed uterus could be misinterpreted as the uterine cavity filled by menstrual blood at HSC. In this case MRI definitely clarified the diagnosis.