The sagittal pelvic tilt angle (A) of 11 patients with rapidly progressive coxopathy (RDC) was calculated using the equations reported by Doiguchi et al. 1992. These were, A=-67.0xL/T+55.7 (male) or A=-69.0xL/T+61.6 (female) based on measurement of the longitudinal (L) and transverse (T) diameters of the pelvic cavity from anterior-posterior radiographs of the pelvis. These were then compared with those of 19 patients with advanced osteoarthritis (OA) hip and of 6 with normal hips selected as age-matched controls.The average pelvic tilt angle with RDC was 34.1±7.2°, which was significantly larger thah the values found with both controls: 26.2±9.1° for the advanced OA and 25.0±2.7° for the normal hip. Results of this study showed that the pelvis of the RDC patient inclines excessively posteriorly which may be associated with the pathomechanism of rapid destruction of the hip joint in this condition. A