Study Design: A prospective instrumental study design. Objectives: In this study, the validity of a device was investigated. Background: Frontal tibiofemoral malalignment is a predisposing factor for developing gonarthrosis. In a previous study a repositioning device was developed to clinically measure frontal tibiofemoral alignment. This device appeared to be more accurate than the conventional method. Case description: This study has examined three types of measurement: conventional method, repositioning device and a bilateral full-leg standing radiograph (Rx). We compared the results of the repositioning device and the conventional method to Rx. To determine the validity of the new repositioning device, n = 41 patients under 18 years of age were included in this study. They all underwent a standing Rx in their history. Outcome: In our study we found a negative correlation between the repositioning device and the already known Rx. Conclusion: The repositioning device was validated, but is not accurate enough to replace Rx. However, physiotherapists can use this device to obtain a more accurate clinical measurement of frontal tibiofemoral alignment.