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Objective: To evaluate three-dimensional (3D) measurements of the lower extremity using a biplanar low-dose X-ray device in children and adolescents. Methods: Firstly, 3D measurements of eight dried bones were analysed by a biplanar low-dose X-ray device (LDX) using stereoscopic software and compared with 3D computed tomography (CT). Secondly, 47 lower limbs of children and adolescents were studied using LDX two-dimensional (2D) and 3D measurements. Both parts were evaluated for femoral and tibial lengths and mechanical angles, frontal and lateral knee angulations, and the femoral neck-shaft angle. Results: The 3D specimen comparison between LDX and CT measurements showed no significant differences: femoral length ( P = 0.069), tibial length ( P = 0.059), femoral mechanical angle ( P = 0.475), tibial mechanical angle ( P = 0.067), frontal knee angulation ( P = 0.198), lateral knee angulation ( P = 0.646) and femoral neck-shaft angle ( P = 0.068). The comparison between LDX 2D and 3D measurements showed significant differences in tibial length ( P = 0.003), femoral mechanical angle ( P < 0.001) and femoral neck-shaft angle ( P = 0.001); other parameters were unremarkable. Conclusions: The 3D LDX system presented reliable measurements compared with 3D CT. Differences between LDX 2D and 3D measurements were noted in the femoral mechanical angle, femoral neck-shaft angle and tibial length. Moderate to good interobserver agreement for the 3D LDX measurements were found. Key Points: • Low radiation dose is essential when assessing potential lower extremity discrepancies • A new biplanar low-dose X-ray device can assess such discrepancies in children/adolescents • This LDX device provides equally reliable 3D measurements as prevalent practice LDX measurements carry good overall interobserver agreement. [ABSTRACT FROM AUTHOR]