The aims of this survey were to establish the familial incidence of Perthes' disease, to note any associated developmental anomalies and to collect information on preceding trauma or synovitis, on the pregnancy and birth, and on various sociological factors. Height and weight measurements were obtained for 217 patients, and comparisons made with those of their parents, unaffected sibs and (local) controls. Results showed an extremely low frequency of Perthes' disease among relatives, with no obvious pattern of inheritance. As genetic factors were not apparent, environmental and sociological causes were sought. The disease occurred particularly in children who were third-born or later in the family, and had older than average parents. Many came from low-income families and one in ten had been a breech birth, shown other malposition or had had a version late in pregnancy. Many children were already undersized at the time of developing Perthes' disease and remained short than average throughout life. Neither their parents nor sibs were shorter than normal, indicating that the patients' short stature was not familial. The child who is going to develop Perthes' disease is already constitutionally and socially at a disadvantage, and during the perinatal period and the first few years of life is perhaps more susceptible to trauma than is a normal child.