The nature and incidence of haematological side-effects of sulphasalazine was sought in a retrospective study of 130 sulphasalazine-treated patients with chronic inflammatory arthritis. Macrocytosis was seen to occur in 27 patients (20.8%) and four patients (3%) developed a macrocytic anaemia. Only eight of 23 macrocytic patients had low red cell folate levels, three of whom were anaemic. An increased risk of developing macrocytosis was seen with doses of sulphasalazine greater than 2 g per day. In most patients the macrocytosis was noted during the first 6 months but did occur in the second and third 6-month period of treatment. Only one patient (0.8%) developed neutropenia and no cases of thrombocytopenia were observed. Regular blood counts should be performed while patients remain on treatment but haematological side-effects are seldom the reason for withdrawal of sulphasalazine.