A 3-year-old boy who had been born prematurely was brought to the emergency department with a 1-day history of intermittent abdominal pain, nausea, and vomiting. He had had no bloody stools or contact with persons known to be sick. His vital signs were normal. An abdominal examination was notable for hypoactive bowel sounds and tenderness in the right lower quadrant. Ultrasonography of the right lower abdomen showed a target-sign lesion measuring 1.7 cm by 1.4 cm (Panel A). A longitudinal view of the same lesion showed a 2.4-cm segment of the small intestine telescoping into itself — a finding known as the “sleeve sign” (Panel B). A diagnosis of ileoileal intussusception was made. Intussusception is the most common cause of bowel obstruction in children. In most cases, small-bowel intussusception reduces spontaneously. In this case, the patient’s symptoms and abnormal findings on ultrasonography abated 30 minutes after the initial ultrasound examination. The patient was monitored in the hospital for 1 day. He had no recurrence of abdominal pain and was discharged. Three days after presentation, a follow-up abdominal ultrasound examination was performed, and no mass serving as a pathologic lead point was identified. As in most cases of pediatric small-bowel intussusception, the patient’s episode was considered to be idiopathic.