The sacro-iliac joint of a patient who had been ill with ankylosing spondylitis for half a year was investigated histologically at necropsy. Two basic histological phenomena were apparent: inflammation and aggressive proliferating chondroid metaplasia at the chondro-osseous junction and in the subchondrium. Joint cartilage degeneration, joint cartilage fusion, capsule ossification and subchondral bone apposition were observed as epiphenomena at the sacro-iliac joint. Both basic morphological phenomena are due to the same cause which has not yet been clarified. They progress independently of each other with varying intensity. This assumption explains differing opinions and the relation of inflammation and pathological ossification of the skeletal axis as well as the unsatisfactory objective results of treatment. Only a combination therapy directed against both morphological basic phenomena will result in an objective therapeutic success and not only in a symptomatic improvement of ankylosing spondylitis.