There is wide variability between subjects in the pharmacokinetic parameters of hydroxychloroquine. A range of concentrations is achieved by individuals receiving the same dosage. In a cross-sectional study, mean hydroxychloroquine concentrations in patients with shorter duration and lower intensity of morning stiffness and no rheumatoid factor were significantly higher than in patients with worse disease activity. Variable kinetic parameters, causing variable concentrations, are likely to be contributing to the variability in response to hydroxychloroquine in rheumatic diseases.