Imperforate hymen is a relatively rare congenital anomaly which may lead to the development of hematocolpos during puberty. Clinical examination reveals imperforate hymen but abdominal ultrasound and tumor markers findings may be misleading. The presented case shows how MRI may contribute to confirm this diagnosis, define the extent of the collection exclude coexisting abnormalities and avoid an unnecessary surgical intervention.A case of an 11 years old girl is presented who was referred to our clinic with the symptoms of pelvic pain and urine retention. Transabdominal ultrasound findings of pelvic mass and an elevated tumor marker Ca 125 contributed to a referral for exploratory laparotomy with possible diagnosis of ovarian malignancy. After admission in our tertiary center a careful history raised the diagnosis of hematocolpos and the clinical examination showed an imperforate hymen. An MRI of the pelvis was performed to confirm the diagnosis of hematocolpos and exclude other coexisting congenital abnormalities. She was managed by crucial hymenectomy and drainage.The diagnosis of hematocolpos should always be considered in the differential of lower pelvic pain in a young adolescent girl. Detailed family history and clinical examination are mandatory to establish a diagnosis. MRI of the pelvis should be performed to confirm the clinical diagnosis, not only to evaluate the extent of the collection and the presence of possible coexisting anatomic variations of the urogenital tract, but also to avoid an unnecessary surgical intervention by laparoscopy or laparotomy.