Sulphasalazine has recently been shown to have an effect in ankylosing spondylitis but the clinical indication for its use is controversial. We have used an 'interventional' study design to investigate the clinical and laboratory effects of sulphasalazine in a group of 20 patients with active ankylosing spondylitis and peripheral joint disease. Following an initial assessment period, patients were treated with sulphasalazine for 24 weeks and the drug was then withdrawn and the patients monitored for a further 12 weeks. Significant improvements were observed in chest expansion, number of active joints, ESR and CRP which deteriorated after withdrawal of sulphasalazine. No change in spinal mobility was demonstrated. The 'interventional' design may be a useful screening procedure for identifying potential second line drugs in ankylosing spondylitis.