We studied the rate of revision in 84 consecutive total hip replacements performed for failed osteosynthesis of femoral neck fractures and compared it with that for primary arthroplasty for osteoarthritis. The age and sex adjusted risk of prosthetic failure was 2.5 times higher after failure of fixation, but all the excess risk was in patients over 70 years of age. There were radiographic signs of loosening of the femoral component at five to 12 years after secondary arthroplasty in six of 33 survivors. In general, the results of secondary replacement were no worse than those obtained after primary arthroplasty for femoral neck fracture. We consider that internal fixation should be the primary procedure: total hip replacement is a safe secondary procedure when osteosynthesis fails.