Sonographic measurements of the volume of blood collecting in hematometrocolpos has been carried out in 6 cases of imperforate hymen. Sonometry gave estimates of the amount of retained blood, which were 4% to 28% more than that actually evacuated. The vaginal collection was 5 times or more than the uterine collection. Ultrasound gives characteristic diagnostic features, and the quantitative capabilities may provide information on volume and on the natural history of this condition.