SUMMARYPlatelet count (PC) was compared with standard acute phase reactants (ESR, plasma viscosity and C-reactive protein) and clinical measurements to assess its usefulness as a measure of disease activity in 165 patients with active rheumatoid arthritis. Although PC was elevated (>400×10¹9⁻¹) in 45% of patients and was seen to fall under the influence of second-line drugs, it was considered to be unsuitable as an indication of disease activity since levels fall for reasons other than disease improvement.