Introduction: Imperforate HYMEN (IH) is one of the simple anomalies in the female genital organs, leading to hydrometrocolpos, and hematocolpos.Although, as association with other congenital anomalies has reported, isolated IH is the most common finding, occurring in 0.1% of girls born at term.Case Report: A 5-month old girl infant with feverand poor feeding was admitted. She treated as a case of Urinary Tract Infection (UTI) regarding to active urine and leukocytosis, supported by isolation of E-coli from a supra-pubic urine sample.The serum creatinine level was 1.2 and 45 mg/ dL, respectively. She was an undernourished infant who born via vaginal delivery, weighing 2.9 kg and no any overt congenital anomaly. The ultrasonographic evaluation revealed a normal left kidney, with mild fullness of collecting system.The right kidney was also not visualized. There was a large midline pelvic cystic mass, could be a hydronephric sac of an ectopic right kidney.The renal dimercaptosuccinic acid scan showed a normal left kidney with no any evidence of renal tissue at the right. The voiding cystourethrography revealed a vesicoureteral reflux grade V at the left, with a large bladder. The Intravenous pyelography (IVP) showed a long segment stricture of distal left ureter resulting left sided hydroureteronephrosis due to pressure effect of the pelvic mass. The right kidney was not visualized. The bladder was pushed right laterally. Regarding to the midline pelvic mass showing in the IVP, her external genitalia was carefully inspected. There was a bulging introitus, so the imperforate HYMEN was diagnosed and HYMENectomy was performed. On cystoscopy, there was no any orifice in the right side of the trigone, bladder neck and urethra. A double J catheter was fixed on the left urinary system. She was discharged 3weeks later in good condition with normal renal function.Conclusions: Careful examination of external genitalia including the HYMEN is an important part of evaluation of any newborn girl and who has UTI, particularly when she has a pelvic mass.The vaginal opening should be fully visible at birth. With early diagnosis, possible urinary complications of IH can be prevented only by a simple HYMENectomy.