The motions and moments in the hip and knee in female patients on the waiting list for knee prosthesis surgery with medial (n = 15) or lateral (n = 15) osteoarthritis (OA) were compared with a control group (n = 15). We hypothesized that not only the kinematics and kinetics of the knee but also of the hip would differ between patients the medial and lateral groups. At midstance, patients with lateral OA showed slightly (2 degrees) more maximal (peak) adduction (p = 0.015) of the hip joint and patients with medial OA had 7 degrees more abduction (p < 0.001) than did controls. In patients with lateral OA, the femur was positioned in about 7 degrees more maximum external rotation (p = 0.001), but femur position did not differ between medial OA and controls (p > or = 0.8). There was a tendency to higher internal hip rotation moment in lateral OA compared to controls (p = 0.021). The maximum values of the internal knee abduction moments were 52% higher in medial OA (p = 0.005) and 63% lower in lateral OA (p < 0.001) compared to controls. Cases with medial OA had 9 degrees more, whereas those with lateral OA had 6 degrees less external tibial rotation than controls (medial vs. lateral OA, p = 0.001). We found an association between presence of lateral OA of the knee and the biomechanics of the hip joint. It remains to be evaluated if the changed biomechanics of the hip joint is a reason for development of lateral OA or an observation that is a result of this disease.