Using clinical, radiological and histologic examinations, 41 cases with the rapidly destructive coxarthrosis (RDC) were studied. (a) The radiological findings of RDC were classified into the following three types; type I suddenly started from the normal hip joint and terminated in the upper outer subluxation with destruction of the head and acetabulum resulting in narrowing of the joint space; type II was clinically very similar to type I. The joint was destructed but remained in the primary acetabulum; type III developed into sudden destruction of the joint during ordinary course of secondary RDC. (b) The histological findings of RDC were classified into those involving avascular necrosis and those involving osteoarthritis. (c) 65% of cases of radiological Type I showed avascular necrosis, 33% in Type II, and 12.5% in Type III. (d) The clinical examinations revealed that most cases have some immunological abnormality. The present study, therefore, indicates that the most common pathogenesis of RDC has a basic condition of avascular necrosis in the femoral head in the elderly, in addition, the accompanying rapid destruction appears to be related to an immunological abnormality.