Objective: To examine whether clinical severity is greater among children receiving attention-deficit/hyperactivity disorder (ADHD) care in primary care compared with those in specialty mental health clinics, and to examine how care processes and clinical outcomes vary by sector across three 6-month time intervals. Method: This was a longitudinal cohort study of 530 children aged 5 to 11 years receiving ADHD care in primary care or specialty mental health clinics from November 2004 through September 2006 in a large, countywide managed care Medicaid program. Results: Clinical severity at study entry did not differ between children who received ADHD care in solely primary or specialty mental health care clinics. At three 6-month intervals, receipt of no care ranged from 34% to 44%, and unmet need for mental health services ranged from 13% to 20%. In primary care, 80% to 85% of children had at least one stimulant prescription filled and averaged one to two follow-up visits per year. Less than one-third of children in specialty mental health clinics received any stimulant medication, but all received psychosocial interventions averaging more than five visits per month. In both sectors, stimulant medication refill prescription persistence was poor (31%-49%). With few exceptions, ADHD diagnosis, impairment, academic achievement, parent distress, and parent-reported treatment satisfaction, perceived benefit, and improved family functioning did not differ between children who remained in care and those who received no care. Conclusion: Areas for quality improvement are alignment of clinical severity with provider type, follow-up visits, stimulant use in specialty mental health, agency data infrastructure to document delivery of evidence-based psychosocial treatment, and stimulant medication refill prescription persistence. (Contains 1 figure and 11 tables.)