In 30 elderly women awaiting hip arthroplasty on account of unilateral osteoarthritis of the hip, walking speed and oxygen consumption were measured during a 12-minute test and the power output was calculated from the stair climbing rate. The results were compared with those for a group of 30 normal women of similar age. An age-related decline in maximal walking speed was observed in both groups. After arthroplasty there was a significant increase in maximal walking speed, particularly among the more disabled patients, with the major gain occurring by three months and a further slight increase by six months. Oxygen consumption returned towards normal values, and both stride length and cadence increased by a comparable degree. Mean power output during stair climbing doubled, and both before and after arthroplasty bore a linear relationship to the maximal walking speed.