To determine its similarity to human spondylarthropathies, we studied murine progressive ankylosis, a spontaneously occurring disorder of joints in mice. Clinically, peripheral joints were inflamed initially, then became ankylosed in a predictable sequence from distal to proximal. Forefeet were involved before hindfeet. Axial joint involvement produced severe spinal ankylosis. Extraarticular manifestations included balanitis and crusting skin lesions. Radiographically, bony erosions and calcification of articular and periarticular tissues were extensive, and vertebral syndesmophytes produced a "bamboo" spine. We conclude that progressive ankylosis is a systemic disease with many clinical and radiographic similarities to human spondylarthropathies, and it may represent a useful animal model for the study of the human diseases.