Four hundred and eight siblings, parents, and grandparents of seventy-eight children from the New England area who had congenital dysplasia of the hip were evaluated, by clinical examination and by measurements of the acetabulum on pelvic radiographs, for the signs and sequelae of congenital dysplasia of the hip. Six siblings and four mothers (representing seven of seventy-eight families) had been diagnosed with congenital dysplasia of the hip during childhood. The other ninety-one siblings were asymptomatic and had no radiographic evidence of dysplasia of the hip. In the adults in these families, acetabular coverage (as measured by the center-edge angle of Wiberg) was no different from that in the control subjects. There was no difference between the study group and the control subjects in the prevalence of osteoarthrosis of the hip or of osteoarthrosis that could be considered secondary to congenital dysplasia of the hip. The results indicate that children born to families that have a history of congenital dysplasia of the hip have a greater prevalence of this problem compared with the general population, but also that examinations of the hip in newborns are effective in detecting congenital dysplasia of the hip in such families. The greater prevalence of congenital disease of the hip among the siblings and mothers in these families is consistent with a multifactorial inheritance. The fact that acetabular development in the family members who did not have congenital dysplasia of the hip was no different from that in the control subjects suggests that acetabular dysplasia, rather than being an inherited abnormality, is secondary to subluxation or dislocation.