Radiographs of 110 patients who had undergone 120 high tibial osteotomies (60 closed-wedge, 60 open-wedge) were assessed for posterior tibial slope before and after operation, and before removal of the hardware. In the closed-wedge group the mean slope was 5.7° (SD 3.8) before and 2.4° (SD 3.9) immediately after operation, and 2.4° (SD 3.4) before removal of the hardware. In the open-wedge group, these values were 5.0° (SD 3.7), 7.7° (SD 4.3) and 8.1° (SD 3.9) respectively, when stabilised with a non-locking plate, and 7.7° (SD 3.5), 9.4° (SD 4.1) and 9.1° (SD 3.8), when stabilised with a locking plate. The reduction in slope (-2.7° (SD 4.1)) in the closed-wedge group and the increase (+2.5° (SD 3.4), in the open-wedge group was significantly different before and after operation (p = 0.002, p = 0.003). In no group were the changes in slope directly after operation and before removal of the hardware significant (p > 0.05). There was no correlation between the amount of correction in the frontal plane and the post-operative change in slope. Posterior tibial slope decreases after closed-wedge high tibial osteotomy and increases after an open-wedge procedure because of the geometry of the proximal tibia. The changes in the slope are stable over time, emphasising the influence of the operative procedure rather than of the implant.