Developmental dysplasia of the hip is a crippling condition that affects children and adults. Identical twin studies support a strong causative genetic component. Although clinical tests for newborns can detect gross malformations, it is the subtle malformations that are often not detected, resulting in early onset osteoarthritis of the hip in adults. As a first step in identifying the causative mutation, we have recruited the largest documented affected family with 71 members spanning generations. Clinical and radiographic signs of developmental dysplasia of the hip are described, and the diagnostic challenge of identifying affected family members is discussed.Variable expression of disease allele is evident in several members of the family and greatly contributes to the diagnostic challenge facing clinicians.