A case of medial patellar instability following Insall's proximal realignment studied pre- and postoperatively by gait analysis is presented. Preoperative gait analysis showed an increased stance time period as well as an increased horizontal braking, heel contact and toe-off vertical peak forces on the affected limb. In our case, gait parameters tended to shift towards a normal value pattern after reconstruction of the lateral retinaculum. We speculate about the importance of the passive restraining structures in patellar stability, in contrast to the role of the muscle function advocated by some authors.