We have reviewed the records of 32 infants whose developmental dislocation of the hip (DDH) was treated by reduction and immobilization in a plaster cast. We examined 50 CT scans from 22 patients. The postreduction radiological studies led to a change of cast in 10 patients and, in five of them, the subluxation shown on CT scan was not seen in an earlier radiograph. Where reduction has been undertaken at the time of arthrography or in an older child, a plain radiograph may be adequate to confirm the position in the cast. In an infant, CT scans give superior information, have acceptable risks, and should be used in association with radiation protection measures.