Six hundred infant hips were examined with ultrasonography and the findings compared to radiography. Twelve hips were dislocated and could be recognized as such on the sonograms. The development of the acetabular roof can be determined through assessment of the preformed, nonosseous acetabular rim. Consequently, dysplasia of the hip can be recognized early. Ultrasonography does not replace clinical investigation, but it does reduce the need for radiography, reducing the exposure of the child to radiation.