This prospective study documents the haematological responses in 300 rheumatoid patients (RA) treated with sulphasalazine (SASP) for between 1 and 9 years. It also examines the effect of SASP on the total white cell and platelet counts over 2 years in relation to disease activity in 80 RA patients. Neutropenia occurred in six (2%) (three severe--neutrophil count less than 0.8 X 10(9)/l) after 3 and 12 weeks. The drug was withdrawn in six immediately and in one patient after 21 months when the neutrophil count fell to 0.7 X 10(9)/l. An additional 11 (3.7%) developed mild or transient leucopenia between 2 weeks and 24 months, and eight continued therapy. Thrombocytopenia occurred in one patient at 18 weeks associated with other reactions. Four with Felty's syndrome developed a further fall in the total WBC associated with thrombocytopenia in two. A rise in mean cell volume was common (72%), and macrocytosis (MCV greater than 98 fl) occurred in 27 (9%). Macrocytic anaemia was rare (less than 1%). All haematological problems were reversible. In 80 patients treated with SASP for 2 years there was a significant fall in the median white cell and platelet counts at 3 months associated with improvement in disease activity.