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We produced a dynamic model of experimental hip dysplasia in growing puppies by cast immobilization of a single hind leg with the knee in extension. Serial roentgenographic studies demonstrated a progressive acetabular dysplasia with subluxation and eventually dislocation of the femoral head. A consistent finding in the dysplastic hip was significant hypertrophy of the ligamentum capitum femoris, both in length and in cross-sectional area. Quantitative studies of blood flow in the femoral head by the hydrogen-washout technique in normal and dysplastic hips demonstrated a significant decrease in blood flow to the dysplastic femoral heads. The dysplastic hips were then placed in various positions of immobilization and blood-flow determinations were repeated. The position of forced abduction significantly reduced circulation in the femoral head, whereas immobilization in flexion consistently resulted in the highest rate of blood flow in the femoral head. These experimental data indicate the potential ischemic hazards of immobilization in the forced abducted position in the treatment of congenital dislocation of the hip. Progressive degenerative changes developed in the articular cartilage of the dislocated joint, with severe reduction of safranin-O staining, fibrocartilaginous metaplasia, and hypocellularity. The degenerative changes in the acetabular cartilage were more severe than those in the femoral head. These findings further emphasize the importance of reducing the dislocated hip at the earliest opportunity in order to prevent irreversible degenerative changes.