BACKGROUND: Judging when it is safe to remove an external fixator or plaster cast requires clinical and radiological assessment, both of which are subjective. Weight bearing has been shown to increase with time post-fracture and we hypothesised that it could be used as an objective measure of fracture healing. METHODS: Ground reaction force (and hence weight bearing) and fracture stiffness were measured serially in a group of 12 patients with tibial fractures treated by external fixation. Ground reaction force was measured for both fractured and non-fractured limbs using a force plate and the fracture stiffness was measured using the Orthometer((R)), a commercially produced device for measuring the stiffness of fractures treated by external fixation. FINDINGS: In 10 patients who made good recoveries, prior to fixator removal, weight bearing though the injured leg was seen to approach 90% of that through the uninjured leg and the fracture stiffness exceeded 15Nm/deg. Two patients with delayed union achieved weight bearing of 40% of normal and a fracture stiffness of less than 5Nm/deg at 20 weeks. INTERPRETATION: Weight bearing correlates reasonably well with fracture stiffness. It is quicker and easier to measure than fracture stiffness and potentially has relevance to other fracture fixation methods.