1. A genetic and orthopaedic analysis of a family of seventy members exhibiting the nail-patella syndrome is described.

2. The disorder is found to be determined by a simple dominant autosomal gene with complete penetrance, but displaying variable expressivity and great pleomorphism.

3. In the family reported all ten of the affected individuals whose blood was typed belonged to group O, demonstrating the adjacent chromosomal position of the nail-patella gene and the ABO blood group locus.

4. The pathogenesis is one of hypoplasia of the nail beds, but hyperplasia in mesodermal tissue as shown by the formation of iliac horns.

5. The skeletal changes are ascribed to a functional disturbance in the development of the traction epiphyses rather than to a primary teratogenic effect.