Aims: Valgus pes planus deformities in patients with RA are similar to those seen with TPTD. The valgus pes planus deformities that derive from RA and TPTD do not necessarily have the same etiology, however. With this in mind, we studied TPTD in patients with RA using plain radiographs and MRI scanning. Methods: We studied 12 feet from 10 patients with RA, with 1 male and 10 female subjects. Plain radiographs were taken using the method of Yokokura, and the L, N, C, R and T values were determined for the longitudinal arch. MRI þndings were then classiþed as Type I, II or III, based on the classiþcation of Conti et al. The radiological þndings were also correlated with the clinical þndings. Results: The mean values for the longitudinal arch parameters were: L 21.2±4.0%; N 26.5±6.2%; C 30.5±4.5%; R 48.9±5.8%; and T 30.8±4.3%. MRI scanning revealed 4 feet of Type I, 3 Type II, and no Type III, with no abnormality detected in 5 feet. Conclusions: We examined the relationship between foot deformities and the tibialis posterior tendon in patients with RA, using plain radiographs and MRI scanning. In RA a condition corresponding to TPTD may lead to a valgus pes planus deformity, but this deformity may also arise from a high degree of joint destruction unrelated to the tibialis posterior tendon. We therefore concluded that the valgus pes planus deformity in patients with RA is not necessarily caused by disorders of the tibialis posterior tendon. We also concluded that degeneration of the tibialis posterior tendon in patients with RA is related to joint inßammation.