The running shoe provides shock absorption and motion control for the runner. The change in shock-absorption properties has been studied as a function of miles run. Different models of running shoes encompassing various manufacturers' models and price ranges were mechanically tested to simulate the repeated heel strikes of running. The energy absorbed by the shoes was determined from the area under the load deformation curve at various intervals to the equivalent of having run 500 miles. Shoes were also tested at similar intervals after having been worn by volunteers during normal training. A difference of approximately 33 per cent in the initial shock absorption was observed in the different shoe models tested. In general, the mechanically tested shoes retained approximately 75 per cent of their initial shock absorption capability after 50 miles and approximately 67 per cent after 100 to 150 miles. Between 250 and 500 miles, the shoes retained less than 60 per cent of their initial shock-absorption capacity. The shoes tested after having been worn during running by volunteers had similar but less severe degradation characteristics. These shoes tended to retain approximately 70 per cent of the initial shock absorption characteristics were apparent based upon either shoe price or manufacturer. In addition, shoes that exhibited superior initial shock absorption capability often had more rapid degradation characteristics.