The magnitude of thrombocytosis and the possible confounding effect of platelet clumping, an in vitro artifact resulting in spuriously low platelet counts, in active rheumatoid arthritis (RA) was evaluated by a prospective survey of 57 consecutive patients, 60% of whom had thrombocytosis. Five cases (9%) of platelet clumping, assessed by H6000 pictures, were found. A low-grade platelet loss in many of the samples anticoagulated by EDTA was suggested by comparison with platelet counts obtained in parallel blood samples anticoagulated by citrate. Thus, the possibility of spuriously low platelet counts due to laboratory artifacts must always be taken into consideration in RA patients. The relation between thrombocytosis and other estimates of disease activity was also studied. The platelet count in citrated blood in active RA was significantly correlated with ESR, acute phase plasma proteins, and neutrophil, basophil and monocyte counts. In a multivariate regression model, however, only the correlation with haptoglobin (p = 0.06) approached significance.