Objective. To determine the reproducibility of frontal plane knee joint laxity measurement through the assessment of intra- and inter-rater reliability coefficients and intra- and inter-rater agreement coefficients. Methods. Two raters independently assessed the laxity of the knee joint in the frontal plane by three repeated measurements. Fourteen days later the assessment was repeated. Complete data were obtained from 20 healthy subjects. Laxity was assessed using a device which consisted of a chair with a free-moving arm that supported the subject's lower leg. Medial and lateral loads were applied, resulting in a varus and valgus movement in the knee joint. The intra- and inter-rater reliability coefficients [intraclass correlation coefficients (ICC)] were estimated, as were the intra- and inter-rater agreement parameters [standard error of measurement (SEM) and minimal detectable difference (MDD)]. Results. Adequate intra-rater reliability (ICC>0.80) was calculated for each rater's measurements of laxity. The inter-rater reliability was less adequate (ICC=0.65) when calculated using the first day's measurements. However, inter-rater reliability was adequate (ICC=0.88) when calculated using the day 14 measurements. The intra-rater measurement error calculated across occasions was 1.3° for individual subjects. This resulted in an MDD of 3.7°. The inter-rater measurement error, i.e. the SEM and MDD, was higher (1.5° and 4.3°, respectively). Conclusions. Intra-rater reliability of knee joint laxity measurement is good. Adequate training of raters establishes the basis for good inter-rater reliability. In clinical trials, it is preferable for one trained rater to perform the laxity measurement. The measurement of knee joint laxity is limited due to its relatively high measurement error in individual subjects; therefore, this measurement should be restricted to group assessment rather than individual patient assessment.