The use of diagnostic ultrasonography and CT in the investigation of the acutely ill pediatric patient has been steadily increasing because of the relative noninvasiveness of these imaging modalities and because of improvements in resolving capabilities and scanning speed of the latest equipment. In the examination of the pediatric patient, special emphasis must be given to immobilization, sedation, or both in those children too young or too ill to cooperate. Close monitoring and observation of the child throughout sedation is important and requires special personnel and equipment. The usual indications are the search for an abscess or occult infection, trauma, and acute renal failure. Because ultrasonography uses nonionizing radiation and is less expensive, it is usually the initial modality of choice for evaluating the pediatric patient. CT is reserved for situations in which greater anatomic detail and lack of compromise by the presence of bowel gas or bone are necessary. These situations include multiple abscess collections, particularly interloop abdominal abscesses, mediastinal masses and collections obscured by overlying lung, osteomyelitis, older patients with meningomyelocele and with spinal deformity when evaluating the urinary tract, and major multiorgan abdominal trauma.