Objective. To estimate the inter- and intrarater reliability and agreement of instrumented knee joint proprioception measurement in subjects with knee osteoarthritis (OA) and healthy subjects; to assess the effect of variations in the measurement procedure on agreement parameters. Methods. Proprioception was measured by a computer-controlled knee angular motion-detecting device in a movement-detecting task. The angular displacement between the starting position and the position at the instant of movement detection by the subject was recorded. Two raters independently assessed knee joint proprioception. After 14 days the assessment was repeated. Complete data were obtained from 24 subjects with knee OA and 26 healthy subjects. The inter-and intrarater reliability coefficients (intraclass correlation coefficients [ICC]) and inter- and intrarater agreement measures (standard error of measurement [SEM] and minimal detectable difference [MDD]) were calculated. Additionally, the effect of changing the velocity of angular displacement and applying headphone music during the measurement on the absolute error (i.e., SEM and MDD) was estimated at the second occasion. Results. Interrater reliability was good in subjects with knee OA and healthy subjects (ICC 0.91 and 0.89, respectively). Interrater agreement was higher in subjects with knee OA than in healthy subjects (SEM 2.13° versus 0.43°, MDD 5.90° versus 1.19°). Intrarater reliability was good in subjects with knee OA and healthy subjects (ICC 0.91 and 0.86, respectively). Intrarater agreement (SEM and MDD) was 2.26° and 6.26° in subjects with knee OA and 0.39° and 1.08° in healthy subjects. The original measurement and the 2 variations in measurement showed comparable measurement errors for subjects with knee OA and healthy subjects. Conclusion. In knee OA subjects and healthy subjects, knee proprioception measurement shows adequate intra- and interreliability. However, the absolute measurement error is rather high. Therefore, this measurement has limited value in the assessment of individual subjects, but can be recommended for scientific research in groups of individuals. © 2007, American College of Rheumatology.