Intra-articular pressure was measured in 94 hips in 81 children with Perthes' disease and various forms of synovitis of the hip. The pressure was elevated in Perthes' disease (3.4 kPa), transient synovitis (17.3 kPa), septic arthritis (20.2 kPa), reactive arthritis (28.0 kPa) and arthritis with urticaria (32.3 kPa). In transient synovitis the mean pressure was only 2.3 kPa in flexion, while a mean of 26.6 kPa was seen in the neutral position and in internal rotation. In children with synovial effusion of the hip, the position of immobilisation should therefore be in flexion of 30-45 degrees with slight external rotation. The neutral position or extension, even with simultaneous traction, may endanger the circulation of the femoral head.