Intra-articular synovial effusion was visualized in different juvenile hip diseases by ultrasonography; 166 hips from 149 children were examined. Joint aspiration of 97 hips confirmed that ultrasonography was more sensitive than conventional radiography in diagnosing effusion. The magnitude of the ultrasonic joint space correlated well with the clinical severity of the disease, the volume of synovial fluid, and the intra-articular pressure. Considerable widening of the ultrasonic joint space was seen in transient synovitis, septic arthritis, reactive arthritis and arthritis with urticaria; moderate widening was seen in some patients with Perthes disease, and symmetrical joint space in patients with nonspecific arthralgia. We conclude that ultrasonography is valuable in the diagnosis and follow-up of synovial effusion of the hip in children.