Objective: Endometriosis is considered an estrogen-dependent disease of women in their reproductive age and characterized by the occurrence of stromal cells and endometrial-like glands outside the uterine cavity. Patient: A report of a 9-year-old premenarcheal girl who was transferred to the Endometriosis Research Center Berlin-Brandenburg Level III (Academic Teaching Hospital) because of cyclic pelvic pain since her 8th year of life. Interventions: History, examination, abdominal ultrasound, laboratory tests, laparoscopic resection of visible lesions. Paraffin-embedded histology (HE staining) and immunohistochemistry. Results: Endometriosis, defined as the presence of stromal tissue and epithelial glands, was confirmed both by HE staining and immunohistochemistry (CD10), respectively. Conclusions: Young pre- or perimenarcheal girls with chronic/cyclic pelvic pain can have endometriosis, and thus the possibility of endometriosis should be included in the differential diagnosis.