Of twenty-three patients more than twenty years old who were treated for a tibial shaft fracture without a concomitant fibular fracture, six (26 per cent) had delayed union or non-union and six (26 per cent) had varus malunion of the fractured tibia. Pain and roentgenographic changes developed in the ipsilateral ankle within two years of injury in two of these six patients with malunion. Of forty-five patients less than twenty years old with similar fractures, one had delayed union and twelve (27 per cent) had varus malunion. Pain in the ipsilateral ankle was observed in two of these twelve patients with malunion. In addition, a bent fibula was observed in thirteen patients who incurred their fractures when they were less than twenty but in no patients who were more than twenty years old at the time of fracture. Clinical observations were corroborated by biomechanical studies on an experimental model. These studies suggested that when the fibula remains intact, a tibiofibular length discrepancy develops and causes altered strain patterns in the tibia and fibula. These may lead to delayed union, non-union, or malunion of the tibia with the sequelae of joint disturbances. The lower incidence of complications in patients less than twenty years old may be due to the greater compliance of their fibulae and soft tissues.