Throughout 1976 orthopaedic surgeons in three regions of England forwarded details of all new patients with Perthes' disease attending outpatient clinics or admitted as inpatients. The incidence in the Mersey region (11.1 per 100 000 children under fifteen years) was twice that in Wessex (5.5) with Trent having an intermediate incidence (7.6). The ratio of male to female incidence varied between the regions as did the age distribution of male cases. These findings point to the importance of environmental factors in the aetiology of the disease, and suggest the need for further epidemiological studies.