A prospective survey was carried out on all cases of irritable hip presenting at the Royal Liverpool Children's Hospital over a period of one year. All children had a radioisotope scan of the hips and were then followed for one year by serial radiography. Five of the 50 children seen during the one year had areas of ischaemia in the capital femoral epiphysis demonstrated on the scan. all five developed radiological signs of Perthes' disease within sic months. The remaining 45 had radiographically normal hips at one year.