Male New Zealand black/KN (NZB/KN) mice spontaneously develop polyarthritis, characterized by destructive damages to the articular cartilage and bone. We assessed effects of diacerein in male NZB/KN mice by radiographic and histopathologic examinations. Diacerein, cyclosporin A or vehicle for the control were orally administered for 7 months, initiating at 8 weeks of age when the arthritis became apparent. At 39 weeks of age, the NZB/KN mice developed polyarthritis in the joints of the forelimbs and hind legs, radiographically characterized by joint space narrowing, deformation of the joint surfaces, and bone erosions. Histopathologic findings showed that the tarsal joints and knee sections from the NZB/KN mice exhibited overt arthritic lesion. Radiographic and histopathologic findings showed that diacerein and cyclosporin A at a dose of 30 mg/kg/day significantly reduced the progression of arthritis. Diacerein shows articular protecting effects against the development of spontaneous arthritis in male NZB/KN mice and diacerein might be useful in the treatment of chronic inflammatory joint diseases during clinical use. Copyright 1999 OsteoArthritis Research Society International.