A classification of tibial shaft fractures based on etiology, morphology, and clinical features is documented in a series of 291 fractures treated by AO/ASIF rigid internal fixation. The fractures were placed in nine main fracture groups, each with three subgroups according to location in the proximal, middle, or distal segment of the shaft. Group A includes all simple fractures, and Group B includes fractures with butterfly fragments and cortical contact between the main proximal and distal fragments. In Group C there is no such contact. For all degrees of comminution, Group 1 includes the spiral fractures and Groups 2 and 3 the nonspiral bending fractures. Group C2 contains segmental and Group C3 crush and high-velocity projectile fractures.