OBJECTIVES: To compare healthcare resource utilization (HRU) and costs among children/adolescents with attention deficit/hyperactivity disorder (ADHD) in Quebec, Canada, who received an atypical antipsychotic (AAP) as either augmentation or alternative therapy to stimulant, before vs. after initiating the AAP.  METHODS: Patients (6–17 years) with ≥2 documented ADHD diagnoses (ICD-9 codes: 314.0–314.9), who used stimulants for ≥30 days and either switched to an AAP or augmented stimulants with an AAP, were identified in Quebec's healthcare database, the Régie de l'assurance maladie du Québec between 03/2007 and 02/2012. Patients with a documented diagnosis for which AAPs are indicated were excluded. All-cause and ADHD-related HRU and costs (from a public payer’s perspective in 2012 Canadian dollars) were compared between the 6-month period before (pre-index) and after (post-index) patients' first AAP prescription claim.  RESULTS: A total of 453 children/adolescents met the inclusion criteria (54.5% switched from stimulants to AAPs and 45.5% augmented stimulants with AAPs). The mean age was 10.4 years (SD=2.5) and 25.4% were female. The most prevalent documented mental comorbidities in the pre-index period were adjustment reaction (7.1%), anxiety disorder (5.1%), and learning disability (4.4%). Risperidone (81.7%) and quetiapine (16.3%) were the most common AAPs initiated. Compared to the pre-index period, patients incurred, on average, more all-cause outpatient visits and costs (3.2 vs. 4.6; $207 vs. $303), prescription fills and costs (13.3 vs. 22.2; $710 vs. $889), total medical costs ($644 vs. $1,096), and total healthcare costs ($1,354 vs. $1,985) (all p<0.05) during the post-index period. Similarly, ADHD-related total healthcare costs ($835 vs. $1,269; p<0.05) were higher during the post-index period; all-cause and ADHD-related total healthcare costs increased by 46.6% and 52.0%, respectively. CONCLUSIONS: Children/adolescents with ADHD, who received an AAP as either augmentation or alternative therapy to stimulant, incurred higher HRU and costs after AAP initiation, mostly through ADHD-related HRU.