Seventy patients, representing all the survivors from a group of 117 consecutive Moore arthroplasties, were examined 2 years after the operation. A high incidence of para-articular ossification was observed. The ossification was associated with impaired function of the hip joint. The separation of periost and fascia from the greater trochanter in McFarland's approach may be responsible for the high incidence of bone formation. This theory is compatible with experimental evidence.