Leucopenia appears to be a more frequent complication of sulphasalazine treatment in rheumatoid arthritis than in inflammatory bowel disease and poses a management problem. In this study leucopenia was found in 20 patients, 14 of whom were participating in prospective studies (252 patients), giving an incidence of 5.6%. Treatment had to be discontinued in half of these patients. Most (14) episodes of leucopenia occurred early in treatment (less than 24 weeks) but some occurred late and sustained monitoring seems necessary. No predictive factors for leucopenia were elucidated. All patients recovered fully with dose adjustment or, in more severe cases, after withdrawal of sulphasalazine and appropriate supportive therapy. The incidence of leucopenia may be higher in Glasgow than in other units in Britain.