In summary, knee OA is clearly associated with some malalignment of the lower extremity. The direction of knee malalignment (varus or valgus) influences which knee compartment becomes involved and likely influences gait mechanics. However, only one study has compared the gait mechanics of people who have been diagnosed with medial knee OA and those of people who have been diagnosed with lateral knee OA. In addition, no studies have examined the effect of these differences on rear-foot mechanics during gait. Therefore, the purpose of this study was to compare frontal-plane gait mechanics at the hip, knee, and rear foot in people with medial knee OA, people with lateral knee OA, and people who were healthy (control group). We hypothesized that, compared with people with lateral knee OA or the control group, people with medial knee OA would walk with increased peak knee adduction, increased knee adduction excursion, and an increased peak internal knee abduction moment. We also hypothesized that, compared with people with lateral knee OA or the control group, people with medial knee OA would exhibit decreased hip adduction, decreased hip adduction excursion, and a decreased peak hip abduction moment. Finally, we expected that people with lateral knee OA would exhibit increased rear-foot eversion, increased eversion excursion, and an increased peak inversion moment and that, for all variables of interest, values in the control group would fall between values in people with medial knee OA and values in people with lateral knee OA.