Disuse osteoporosis following tibial fractures is associated with pain and prolonged rehabilitation. In a prospective study, 22 patients with stable tibial fractures treated in plaster of Paris were compared with 15 patients in whom unstable tibial fractures were treated by external fixation. All patients were allowed to bear full weight from the day after the injury. The patients treated with plaster of Paris had the less severe fractures but still had a greater number of the higher grades of osteoporosis. We found a significantly lower degree of osteoporosis in the external fixator group. A comparison of the median percentage bearing weight in the two groups of patients showed greater percentage bearing weight in the external fixator group at four and eight weeks after the injury. We suggest that the reduction of post-traumatic osteoporosis is due to improved early function and weight bearing in patients treated by external fixation.