Radium (224Ra) is commercially available again for the treatment of ankylosing spondylitis. Twenty patients suffering from ankylosing spondylitis were treated with weekly intravenous (i.v.) injections of 1 MBq 224Ra for 10 weeks. Therapeutic effect was measured by C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and full blood count, as well as a completion of the Bath ankylosing spondylitis functional index (BASFI) questionnaire. Follow-up was done after three and six months. At the end of the treatment course pain and movement restrictions had improved subjectively in 12 out of 20 patients. These patients were also able to discontinue or reduce their analgesic or anti-inflammatory medications. Subjective improvement was well correlated with a reduction of CRP by 45% and BASFI by 73%. At the six-month follow-up, ten patients reported a lasting improvement, whereas two had suffered a relapse. A late therapeutic response after three months was seen in a single patient only. Patients who did not respond to radium had lower initial levels of acute-phase reactants and peripheral joint involvement. Only mild side-effects, e.g. temporary worsening of pain, were observed. Leukocytes and platelets reversibly decreased by 25%, respectively. It is concluded that 224Ra is an effective and safe treatment for ankylosing spondylitis.