Objective: To evaluate care transition and medication use in young adults with ADHD in Sweden. Method: Observational retrospective study of patients with ADHD from the Swedish National Patient Register, Prescribed Drug Register, and Cause of Death Register (2018-2020). Last contact with pediatric psychiatric care, first contact with adult psychiatric care, and medication use were described for ages 18 to 21 years, inclusive. Results: Of 19,233 patients who had dispensed ADHD treatment, 85.8% had received adult psychiatric care by 21 years of age. The proportion of patients on medication was highest at age 18 years (80.1%), decreasing to 36.1% by age 21 years. By 21 years, there were significantly more patients on versus off ADHD medication who had received healthcare for autism, been prescribed selective serotonin re-uptake inhibitors, and received adult psychiatric outpatient visits (all p < 0.0001). Conclusion: This study demonstrates declining ADHD medication use during the pediatric-to-adult care transition for patients in Sweden.