A method to classify the degree of ectopic-bone formation about the hip following total hip arthroplasty revealed that 21 per cent of 100 consecutive patients treated by total hip arthroplasty had ectopic-bone formation about the hip of various degrees when reviewed six months following the operation. Ectopic-bone formation, however, did not seem to affect the functional result as judged by the Harris hip evaluation unless apparent bone ankylosis resulted.