The preliminary results are presented from a study in which 85 serious tibial fractures were treated with external skeletal fixation. In group I patients were treated with highly rigid fixation. In group II the same fixation was used but axial micromovement was applied across the fracture site for 30 min per day, starting 1-3 weeks after injury and continuing until partial weight-bearing, which leads to self-induced movement. The overall mean time to independent weight-bearing was longer in group I than group II (p = 0.02); delayed union occurred in more group I patients. Objective measurement of fracture stiffness was made by means of strain gauges placed on the fixation frame for 49 fractures treated consecutively. The time to reach stiffness levels equivalent to clinical union was significantly longer in group I than in group II. The fractures in the treatment groups were of comparable severity. It seems that the fracture healing process is susceptible to small changes in mechanical environment.