Use of larger diameter femoral heads has been popularised in total hip arthroplasty (THA). Recent studies have implicated larger femoral heads in early failure. We evaluated what effect the size of the femoral head had on the early functional outcome in order to determine the optimal head size for the maximal functional outcome. There were 726 patients who underwent elective THA and were divided into 3 groups according to head size then compared with respect to functional outcome scores and dislocation rates. This study failed to show that increasing the size of the femoral head significantly improved the functional outcome at 1 year after total hip arthroplasty but that the use of a 36 mm or greater femoral head did reduce the dislocation rate.