Excessive foot pronation has been speculated to be a cause of leg and foot problems among runners. Foot orthotic devices are often used to modify this condition. Examination of the records of 180 patients treated for various running injuries showed that 83 individuals (46%) were prescribed orthotic devices and that 65 of these runners (78%) were able to return to their previous running programs. In order to assess further the effects of this type of orthotic device, six runners were selected from this group and filmed using two cameras (200 frames/sec) under three conditions: (1) barefoot, (2) regular shoe, and (3) regular shoe plus orthotic device. Both the period of pronation and the amount of maximum pronation were significantly reduced by using the foot orthotic device. The data support the conclusion that foot orthotic devices can be successfully used to modify selected aspects of lower extremity mechanics during the support phase of running.