Perthes' disease is thought to be rare in black children but no figures of incidence have previously been available. A search was therefore made for cases of the disease occurring in the Eastern Cape during the five-year period 1975 to 1979. Of 55 cases found, 38 were in white children, 11 in children of mixed ancestry ("coloured" children) and six in black children. The annual incidence in white children aged 14 years and under was 10.8 per 100 000, in coloured children 1.7 per 100 000, and in black children 0.45 per 100 000. In all races the incidence in the metropolitan area of Port Elizabeth was roughly twice that in the rural part of the region. The reason for the low incidence in coloured and black children is not known but various factors are suggested.