The aetiology of Perthes' disease of the hip, avascular necrosis of the head of the femur, is unknown. Children with the disease have a generalised abnormality of growth. A similar disproportionate growth is found in chicks given a diet deficient in manganese. In Liverpool, which has the highest incidence of Perthes' disease reported anywhere in the world, children with the disease were shown to have lower blood manganese levels than controls. This is evidence that manganese deficiency around the time of birth may be a cause of Perthes' disease, but the results require confirmation.