The aim of this study was to describe the pediatric population with ADHD and their pharmacological treatment. Using the Swedish National Patient Register and the Prescribed Drug Register we identified individuals below 19 years of age who were diagnosed or medically treated for ADHD for the first time 2006–2007. The unique patient identifiers were used to link information from the two registers to describe demographic characteristics, hospital care and drug treatments. Logistic regression model estimated the association between age, sex, frequency of hospitalization, diagnosis or treatment for other mental disorders and risk of gap in the treatment. Totally the study included 7931 patients of whom 74% were males. The mean age at first diagnosis was 12 years. Some 84% were medically treated for ADHD and approximately 90% received methylphenidate as the first substance. Combination therapy was rare and the most common combination was methylphenidate and atomoxetine. More than 55% of the patients, which could be followed up for two years after start of treatment, had at least one treatment gap of six months. Older age at diagnosis, lower number of hospitalizations and comorbidity with other mental disorders increased risks of gaps in medication. Approximately one fifth of the patients recorded in the National Patient Register as diagnosed with ADHD did not receive pharmacological treatment. Medication adherence seems to be low, when measured as gaps in treatment.