SUMMARY
Transverse vaginal septum, rare defect of mullerian duct is a congenital anomaly of female genital tract resulting in abnormal collection of menstrual blood in vagina and uterus i.e. hematometrocolpos. Septum may lie at any level in vagina but most commonly lies at uper1/3rd 46% and least likely in lower 1/3rd of vagina i.e. 14%. Mostly patients present with primary amenorrhoea, cyclical abdominal pain and some time abdominal mass and therefore easily diagnosed but problem arises when patient present with atypical symptoms like urine retention, constipation and leg pain. Here we report a case of 15 years old girl who presented in emergency department with acute retention of urine for seven hours. Ultrasound Pelvis showed huge cystic mass in pelvis and MRI pelvis showed distended uterus and vagina. So hematometrocolpos was diagnosed, examination under anaesthesia revealed thick vaginal septum1cm away from introitus, no bulging in hymen was noted. Septum was surgically resected and 2.5liters (2500ml) chocolate coloured fluid was drained.