We have examined 50 persons with sclerosteosis in the Afrikaner community of South Africa and 15 individuals with van Buchem disease in Holland. The clinical and radiographic manifestations of these conditions are very similar, the only notable differences being greater severity and syndactyly in the majority of the patients with sclerosteosis. The Afrikaners have Dutch antecedants and it seems likely that these autosomal recessive disorders result from homozygosity of the same faulty genes. The phenotypic variation may be due to the epistatic effect of modifying genes in the Afrikaner population.