Sulphasalazine (SASP) is now accepted as an effective slow-acting antirheumatic drug for treating active rheumatoid arthritis (RA), but has not been previously evaluated in psoriatic arthritis. An earlier open study suggested that it was well tolerated and potentially beneficial. The present double-blind placebo-controlled trial of 30 patients has now confirmed its efficacy. Greater improvement occurred in those patients on active treatment than on placebo, with more benefit being detected in those patients with the symmetrical polyarticular but seronegative pattern of arthritis associated with a high acute-phase response. SASP was stopped in 26% because of side-effects but these were mild. No exacerbation or remission of psoriasis was observed. Further studies are in progress to determine the degree of efficacy of SASP in different clinical subgroups of psoriatic arthritis.