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To investigate the cause of valgus deformity of the hindfoot in patients who have rheumatoid arthritis and to characterize the effects of the deformity on gait, two groups of patients were evaluated clinically, radiographically, and with gait analysis in the laboratory. Group 1 consisted of seven patients who had seropositive rheumatoid arthritis and normal alignment of the feet and Group 2, of ten patients who had rheumatoid arthritis and valgus deformity of the hindfoot. In Group 2, the disease was of longer duration and the feet were more painful than in Group 1. There was no evidence of muscular imbalance, equinus contracture, valgus deformity of the tibiotalar joint, or isolated deficiency of the tibialis posterior (such as weakness, tenosynovitis, or rupture of the tendon) that could have contributed to the development of the valgus deformity. In the patients who had valgus deformity, quantitated electromyography demonstrated that the intensity and duration of activity of the tibialis posterior was significantly increased, apparently in an effort to support the collapsing longitudinal arch of the foot. Gait studies revealed decreases in velocity, stride length, and single-limb-support time, as well as delayed heel-rise in both groups, but the decreases were more marked in the patients who had valgus deformity. The results of this study suggest that valgus deformity of the hindfoot in rheumatoid patients results from exaggerated pronation forces on the weakened and inflamed subtalar joint. These forces are caused by alterations in gait secondary to symmetrical muscular weakness and the effort of the patient to minimize pain in the feet. Radiographs also suggested an association between the valgus deformity of the feet and valgus deformity of the knees in patients who have rheumatoid arthritis.