A 19-year-old woman presented to the emergency department with a 7-year history of periodic lower abdominal and pelvic pain. She reported no fever, chills, or dysuria. She had not had the onset of menses and had no history of vaginal intercourse. Physical examination revealed a distended lower abdomen, with a firm and tender suprapubic mass extending from the pelvis to the umbilicus (Panel A), and an imperforate hymen (Panel B). Computed tomography of the pelvis revealed a vaginal mass that measured 26.0×13.0×10.4 cm, with upward displacement of the uterus (Panel C). “Hematometrocolpos” refers to the dilatation of the vagina and the uterus with retained menses owing to an imperforate hymen. Patients typically report primary amenorrhea with cyclic monthly abdominal and pelvic pain. This patient was treated with a hymenotomy, which drained 3 liters of brown-colored, malodorous blood. Oral antibiotics were administered to treat possible vaginal infection, and the patient was discharged after 3 days. In outpatient follow-up during the subsequent year, the patient’s abdominal and pelvic pain had resolved, and pelvic ultrasonography confirmed that the uterus, vagina, and adnexa were normal.