A relatively rare congenital malformation, hematocolpos is the progressive accumulation of menstrual blood in the vaginal cavity at puberty. It is often the consequence of a hymen imperforation. It is clinically manifested by cyclic pelvic pain and primary amenorrhea and, more rarely, by a pelvic mass syndrome. The diagnosis is primarily clinical. Ultrasound and magnetic resonance imaging (MRI) are additional tests to confirm hematolcolpos and exclude other associated genitourinary malformations. Treatment consists of a hymenotomy to drain the hematocolpos. The age of discovery of hematocolpos varies from 13 to 14 years. We report a case of hematocolpos secondary to hymenal imperforation diagnosed in a 17-year-old girl with periodic pelvic pain, primary amenorrhea and a pelvic mass. A first ultrasound trap had evoked a large ovarian cyst but the repeat pelvic ultrasound in our hospital confirmed the diagnosis of hematocolpos.