The objective of the study was to compare two methods (Ranawat's and Pierchon's) used to determine the centre of rotation of the hip and establish which method calculates a position nearer to the real centre of rotation.We selected 24 patients with unilateral osteoarthritis of the hip. The centre of rotation of the healthy hip was determined in two consecutive radiographic studies by superimposing a template of circles and using two axes as the reference lines (X-axis=teardrop line; Y-axis=a line perpendicular to the X-axis, drawn from the intersection of the ilio-ischiatic line and the teardrop line). After ensuring the stability of these references, both methods were applied to the same radiograph to determine which one established a centre of rotation nearer to the anatomical centre identified by the template of circles.When the values for the healthy hip are compared with those obtained using Ranawat's method, highly significant differences are observed for both X (P<0.0001) and Y (P<0.0001). When the results for the healthy hip are compared with the values obtained using Pierchon's method, neither the X (P=0.722 ) nor the Y values (P=0.112) show any significant differences. It would be advisable to use Pierchon's method to determine the centre of rotation during the preoperative planning for a total hip arthroplasty when the anatomical alteration is bilateral.