Serdexmethylphenidate/dexmethylphenidate (SDX/d-MPH) is approved for the treatment of patients aged ≥6 years with attention-deficit/hyperactivity disorder (ADHD). A 12-month, open-label safety study with SDX/d-MPH in children with ADHD showed that SDX/d-MPH was well tolerated and comparable with other methylphenidate products. In this analysis of the 12-month study, the objective was to characterize the effect of SDX/d-MPH on growth in children with ADHD over 12 months. This was a analysis of a dose-optimized, open-label, phase 3 safety study of SDX/d-MPH in children aged 6-12 years with ADHD (NCT03460652). Weight and height -score analyses were conducted. -score change from baseline was calculated based on the baseline values for the subjects remaining in the study at the observation time point. Subjects (  = 238) from the treatment-phase safety population included all enrolled subjects who received ≥1 dose of study drug and had ≥1 postdose safety assessment. During treatment, the mean weight and height -scores decreased over time from their respective baselines. At the 12-month time point, mean (standard deviation SD) -score changes from baseline for weight and height for the subjects remaining in the study were -0.20 (0.50) and -0.21 (0.39), respectively; however, these mean changes in Z-scores were not clinically significant (change <0.5 SD). Long-term treatment with SDX/d-MPH was associated with modest reductions in expected weight and lower-than-expected increases in height: effects that plateaued or diminished later in treatment. The overall effects of SDX/d-MPH on growth velocity (the change in weight and height from one time point to the next) were minimal, and the range of changes was not considered clinically significant. ClinicalTrials.gov identifier: NCT03460652.