A 17-year-old girl with colicky abdominal pain and chronic anemia presented to the gastrointestinal service of the University Hospital of Essen. In the routine workup, there were no pathological findings despite the anemia. Because of the fluctuation of symptoms with a climax at the time of menstruation, consecutive ultrasound studies were performed revealing a visible mass inside the gallbladder. This finding was confirmed by a magnetic resonance imaging (MRI) study performed at the same time. Because of the severe anemia by that time, a cholecystectomy was performed, and histology reconfirmed the diagnosis of isolated gallbladder endometriosis. The patient recovered well and has had no recurrence of the disease to date.