A one-month-old girl was admitted to the hospital because of an intraabdominal mass.After a 42-week gestation, the infant had been delivered by cesarean section because of cephalopelvic disproportion. There were no complications during the pregnancy or delivery. A routine ultrasonographic examination ( Fig. 1), performed during the 28th week of pregnancy, showed a multiseptated cystic mass in the midabdomen. An ultrasonographic study during the 29th week of pregnancy showed the same mass slightly to the left of the midline. Another prenatal ultrasonographic examination at 30 weeks also showed the cystic abdominal mass near the midline. The infant weighed 3.76 kg at birth. She was fed a commercial formula with iron and was healthy from birth, although her parents described her as "colicky." Occasional constipation was noted; she did not vomit. Thirteen days before admission a computed tomographic (CT) scan of the abdomen (Fig. 2) revealed a cystic mass, 5 by 5 cm, that displaced the bowel to the left side and appeared to be unrelated to the genitourinary system or the liver. She was admitted to the hospital.The heart rate was 160, and the respirations were 24. The weight was 4.15 kg.On physical examination the infant appeared well. The head was normal, and the neck was supple. The lungs and heart were normal. The abdominal examination was normal; no organs or mass was felt. The external genitalia were normal, and rectal examination disclosed no pelvic or sacral mass. The neurologic examination was normal.The hematocrit was 34.9 percent, the white-cell count 13,200 per cubic millimeter, and the platelet count 505,000 per cubic millimeter. The mean corpuscular volume was 84 m m 3 .A diagnostic procedure was performed.