Objective: To evaluate the treatment effect size throughout the day of amphetamine extended-release oral suspension (AMPH EROS; Tris Pharma, Inc., Monmouth Junction, NJ, USA) in a laboratory classroom study conducted in children aged 6–12 years with attention-deficit/hyperactivity disorder (ADHD). Methods: A post hoc analysis was performed to assess the overall effect size as well as the effect size at each time point from early morning through evening (1, 2, 4, 6, 8, 10, 12, and 13 hours postdose) for each efficacy measure evaluated in a 5-week, randomized, dose-optimized, double-blind, placebo-controlled, laboratory classroom assessment, efficacy, and safety study of AMPH EROS ( N  = 99). Change from baseline of the primary (Swanson, Kotkin, Agler, M-Flynn, Pelham SKAMP-C) and key secondary (secondary efficacy assessments included the SKAMP attention SKAMP-A, SKAMP-deportment subscale SKAMP-D, Permanent Product Measure of Performance-number of problems attempted PERMP-A, PERMP-number of problems correct PERMP-C) efficacy measures were analyzed using a linear mixed model repeated-measures analysis model. Comparisons among treatments were adjusted for multiple comparisons using the Bonferroni method. The effect size was estimated using Cohen's d , to determine “small,” (0.2), “medium,” (0.5), or “large” (0.8) magnitudes of treatment effects. Results: Large overall effect sizes were observed for all primary and key secondary efficacy assessments. Moreover, the SKAMP-C, PERMP-number of problems attempted, and PERMP-C scores showed large effect sizes at each time point evaluated across the day, from 1 to 13 hours postdose. The SKAMP-A and SKAMP-D scores showed a medium to large effect size at each time point. Conclusions: AMPH EROS demonstrated a large and consistent effect size across the day, including early in the morning, in the treatment of symptoms of ADHD in children aged 6–12 years. Trial Registration: clinicaltrials.gov identifier: NCT02083783