This study was designed to evaluate the reliability and reproducibility of frontal plane malalignment measurements using the mechanical axis deviation method in achondroplasia and to determine whether the patient's age has any influence on these measurements. A total of 150 anteroposterior standing radiographs of the lower extremities were randomly selected for the study. Radiographs were divided into three groups according to age: group 1, younger than 6 years of age; group 2, 6 to 10 years of age; group 3, older than 10 years of age. Interobserver agreement for the medial proximal tibial angle and the lateral distal tibial angle measurements were poor (0.32 and 0.38, respectively) in group 1, but agreement increased between observers with increasing patient age. Good to excellent intraobserver reproducibility was found in all groups, except measurement of the medial proximal tibial angle in group 1, where the results were poor (0.36). Significant measurement errors in the proximal and distal tibial joint lines are possible in children less than 6 years of age with achondroplasia.