A 10-year-old girl was admitted to the hospital because of urinary retention.The patient had been in excellent health until two days earlier, when she experienced suprapubic "pressure" on voiding, with urinary frequency. The next day, the discomfort was so severe that she was unable to void. Examination at this hospital showed distention of the bladder to the umbilicus. An indwelling catheter was inserted and 600 ml of urine drained. A urine specimen was positive (+) for protein and had a pH of 7.0 and a specific gravity of 1.021. The sediment contained 4 red cells and 1 white cell per high-power field. A portion of the specimen was sent for culture. A radiograph of the abdomen revealed a pelvic soft-tissue opacity, a finding consistent with enlargement of the bladder; no calcifications were seen. The catheter was removed, and the patient was discharged with instructions to take trimethoprim-sulfamethoxazole and phenazopyridine. That evening she became unable to void and returned to this hospital. An indwelling catheter was again inserted, and she was admitted.The patient had been born in this country to an Asian couple who had immigrated to the United States. She was premenarchal. All the members of her family were healthy. Her immunizations were up to date. There was no history of difficulty with urination, urinary tract infection or risk factors for it, fever, chills, sweats, abdominal or back trauma, vomiting, flank pain, gross hematuria, weakness of the legs, gait ataxia, or weight loss.The temperature was 37.3°C, the pulse was 80, and the respirations were 20. The blood pressure was 125/90 mm Hg.On examination, the girl appeared well. No rash or lymphadenopathy was found. No costovertebralangle tenderness was noted, and the femoral pulses were normal. The external genitalia were normal, and the hymen was intact. The results of a rectal examination were normal, as was anal tone. The results of hematologic laboratory tests are shown in Table 1. The blood chemical values were normal.Specimens of urine were obtained for culture, and trimethoprim-sulfamethoxazole and phenazopyridine were continued. On the second hospital day, the temperature rose to 37.4°C but was subsequently normal. An ultrasonographic study (Fig. 1) showed