Objective: There is a need for the routine monitoring of treated attention-deficit hyperactivity disorder (ADHD) for timely policy making. The objective is to report and assess over a decade the prevalence and incidence of diagnosed ADHD in Canada. Methods: Administrative linked patient data from the provinces of Manitoba, Ontario, Quebec, and Nova Scotia were obtained from the same sources as the Canadian Chronic Diseases Surveillance Systems to assess the prevalence and incidence of a primary physician diagnosis of ADHD (ICD-9 and ICD-10 codes: 314, F90.x) for consultations in outpatient and inpatient settings (Med-Echo in Quebec, the Canadian Institute of Health Information Discharge Abstract Database in the 3 other provinces, plus the Ontario Mental Health Reporting System). Dates of service, diagnosis, and physician specialty were retained. The estimates were presented in yearly brackets between 1999-2000 and 2011-2012 by age and sex groups. Results: The prevalence of ADHD between 1999 and 2012 increased in all provinces and for all groups. The prevalence was approximately 3 times higher in boys than in girls, and the highest prevalence was observed in the 10- to 14-year age group. The incidence increased between 1999 and 2012 in Manitoba, Quebec, and Nova Scotia but remained stable in Ontario. Incident cases were more frequently diagnosed by general practitioners followed by either psychiatrists or paediatricians depending on the province. Conclusion: The prevalence and incidence of diagnosed ADHD did not increase similarly across all provinces in Canada between 1999 and 2012. Over half of cases were diagnosed by a general practitioner.