Objective. To examine the association of serum CRP and ESR with muscle strength in patients with knee OA.Methods. Cross-sectional data from 285 patients with knee OA from the Amsterdam Osteoarthritis (AMS-OA) cohort were analysed. CRP (mg/l) and ESR (mm/l) were measured in serum from patients' blood samples and the values were dichotomized for the analyses. Strength of quadriceps and hamstring muscles was assessed using an isokinetic dynamometer. Univariable and multivariable linear regression analyses were used to assess the association of CRP and ESR with muscle strength, adjusting for relevant confounders.Results. Elevated levels of serum CRP (β = -0.10; P = 0.04) and ESR (β = -0.12; P = 0.02) were associated with lower muscle strength after adjustment for age, sex, comorbidities and NSAID use. The associations were no longer significant when BMI was incorporated in the adjusted model.Conclusion. Inflammation might influence muscle strength in patients with knee OA. Moreover, the link between inflammation and obesity might explain the effect that BMI has in the associations between inflammatory markers (i.e. CRP and ESR) and muscle strength. © The Author 2012.Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved.