We have determined the natural history of hip development in 42 patients with multiple epiphyseal dysplasia (MED). Premature osteoarthritis was a frequent outcome and was almost inevitable before the age of 30 years in those with incongruent hips. There were two types of immature hips: type I, the more severe form, had a fragmented and flattened ossific nucleus and acetabular dysplasia, was misshapen at skeletal maturity and osteoarthritic by 30 years of age; the milder type II hip had a small, rounded, uniformly ossified nucleus and a more normal acetabulum. Type II hips were well formed at maturity and were less prone to premature osteoarthritis. Considerable variations were noted in the manifestations of MED between families but not within families. The prognosis of a child's hip could be predicted; in sporadic cases from the type of immature hip, and in familial cases by also taking into account the outcome of affected relatives.