After intravenous injection of 99mTc methyldiphosphonate, 190 hip bone scans were performed on 103 children. In 64 children with Perthes disease, 61 of 71 hips (86%) showed abnormal scintigraphy. There were no false positives. In other hip disorders, the scan was never considered to simulate Perthes disease. There were 36 children with acute synovitis and three with Meyer's dysplasia. The sensitivity was 97%, suggesting that this test should be used more frequently in the early assessment of hip pain in the young child, before the appearance of characteristic radiographic abnormalities. This test is also valuable for follow-up of the disease because of its prognostic value--the bone scan becomes normal far earlier than the radiograph. The progressive normalization of the scan may help determine when the child can begin weight-bearing.