After an average of 5 years following therapy with Ra224, the concentration of Tc99m methyl diphosphonate (MDP) in the region of the sacroiliac joints and lumbar spine was measured in 15 patients with ankylosing spondylitis. The findings were supplemented by those of clinical, pathological, and roentgenological examinations. The concentration of MDP was normal in the sacroiliac joints in almost all the patients, but was raised in the lumbar spine in two-thirds. This is to be interpreted as indicating enhanced bone turnover, the difference from that in healthy subjects being highly significant (p less than 1). Increased ossification of the lumbar spine could be demonstrated roentgenologically in two-thirds of the patients. Differential blood count, blood sedimentation rate, and electrophoresis remained unchanged after therapy. However, in 60% of patients the mobility of the spine was the same or even improved; 46% felt better after Ra224 treatment than before it.