BACKGROUND We assessed the value of measuring biomechanical stiffness by assessing the fixator's external deformation as an objective means for monitoring fracture healing and determining the postoperative treatment regime, as compared to clinical and radiographic means of evaluation. METHODS One hundred patients with tibial shaft fractures managed by unilateral external fixation had their fracture stiffness monitored. Stiffness was measured and clinical and radiological examinations were performed every 3-4 weeks. RESULTS The time required for healing as indicated by stiffness measurement was an average of 2.5 weeks earlier than by radiological assessment. Eighty-two patients healed within 19 weeks (12.1+/-3.3 weeks) and ten patients in the following 6 weeks (24+/-4.3 weeks). Eight patients did not show an increase in fracture stiffness and received intramedullary nailing at a second operation. The average healing time was 11.3+/-4 weeks for type A, 13.1+/-3.6 weeks for type B fractures, and 15.1+/-5.9 weeks for type C fractures. The healing time for closed fractures was 11.3+/-3.2 weeks and for open fractures 14+/-4.9 weeks. CONCLUSIONS The measurement of fracture stiffness allows the detection of patients at risk for nonunions. The healing time increased with increasing fracture gap size and was less in patients with younger age, less complex fractures, and lesser degrees of soft tissue damage.