We report on a comparative measurement of anterior knee joint laxity on 30 healthy test subjects and 30 patients with anterior knee joint instability using a new knee testing device (Rolimeter) and the KT- 1000 arthrometer compared to manual assessment of the anterior tibial translation in 30 degrees flexion (Lachman's test). We wanted to establish the reliability of the Rolimeter device compared to manual assessment and to the KT- 1000 arthrometer. The statistical evaluation by means of an analysis of variance (ANOVA) at a significance level of 5% showed no significant difference in the exactitude of measurement between the Rolimeter and the KT-1000 arthrometer, but altogether a significant difference between both instrumental methods compared with manual assessment. Thus the Rolimeter provides an economic, exact and simply operating device for quantifying anterior knee joint instability.