We assessed leg length discrepancy and hip function in 90 patients undergoing primary total hip arthroplasty before surgery and at three and 12 months after. Function was measured using the Oxford hip score (OHS). After surgery the mean OHS improved by 26 points after three months and by 30 points after 12. After operation 56 (62%) limbs were long by a mean of 9 mm and this was perceived by 24 (43%) patients after three months and by 18 (33%) after 12. The mean OHS in patients who perceived true lengthening was 27% worse than the rest of the population after three months and 18% worse after 12. In 55 (98%) patients, lengthening occurred in the femoral component. Appropriate placement of the femoral component could significantly reduce a patient's perception of discrepancy of length.