OBJECTIVES:  Despite availability of several treatment options, adequate symptom control remains a major concern in attention-deficit/hyperactivity disorder (ADHD). Lack of symptom control may impose a significant economic burden, yet few studies have quantified the frequency of uncontrolled symptoms and its relation to costs. This retrospective claims database analysis assessed the impact of ADHD symptom control on health care costs. METHODS:  MarketScan® Commercial Database claims between January 1, 2010 and June 30, 2015 were used to identify pediatric (age 6-12), adolescent (13-17), and adult (18+) patients with ≥2 ADHD diagnoses (ICD-9 314.0x), ≥1 newly-started ADHD medication pharmacy claim, and continuous enrollment 6 months before and 12 months following ADHD medication initiation ("index"). Symptom control cohorts were defined from 6-month post-index treatment changes: (i) “well controlled”—without dose increase or treatment switching/augmentation; (ii) “partially controlled”—dose increase; and (iii) “poorly controlled”—dose increase and/or treatment switching/augmentation. Annual adjusted cost differences were estimated using generalized linear models. RESULTS:  The ADHD patient sample (97,230 pediatric; 58,641 adolescent; 135,177 adults) was 69.7%, 65.0%, and 48.7% male, respectively. Mean (SD) age was 8.9 (1.9), 15.0 (1.4), and 31.2 (12.1) years for the pediatric, adolescent, and adult groups, with percent well- (62.1%, 73.7%, 73.0%), partially- (8.8%, 6.4%, 6.1%), and poorly-controlled (29.1%, 19.9%, 20.9%), respectively. Well-controlled pediatric patients had lower annual mean total costs ($3,709) than partially- ($4,269) and poorly-controlled patients ($5,127) (all p-values <0.001). Annual mean medical and pharmacy costs were also lower among well-controlled patients ($2,180, $1,572, respectively) than partially- ($2,163, $2,123) and poorly-controlled ($2,776, $2,363) patients. Similar cost trends were observed for adolescent and adult populations. CONCLUSIONS:  Our findings suggest that, after one year of treatment, 20.9% - 29.1% of ADHD patients were poorly controlled, and incurred 20.7% - 38.2% greater costs than well-controlled patients, suggesting better symptom control may have economic benefits.