Objective: To describe the clinical course of attention-deficit/hyperactivity disorder (ADHD) symptom severity and diagnosis from ages 3 to 5 up to 9 to 12 years during a 6-year follow-up after the original Preschool ADHD Treatment Study (PATS). Method: A total of 207 participants (75% male) from the original PATS, assessed at baseline (mean age, 4.4 years, when all met criteria for ADHD) and 3 months later (before medication treatment), were re-evaluated in three follow-up assessment visits (year 3, mean age 7.4 years; year 4, 8.3 years; and year 6, 10.4 years). Parents and teachers rated symptom severity, and clinicians established psychiatric diagnoses. Analyses examined longitudinal changes in symptom severity and ADHD diagnosis. Results: Parent- and teacher-rated symptom severity decreased from baseline to year 3 but remained relatively stable and in the moderate-to-severe clinical range through year 6. Girls showed generally steeper decreases in symptom T-scores. At year 6, 89% (160/180) of remaining participants met ADHD symptom and impairment diagnostic criteria. Comorbidity of oppositional defiant disorder and/or conduct disorder was associated with a 30% higher risk of having an ADHD diagnosis at year 6 in the multiple logistic model. Medication status during follow-up, on versus off, did not predict symptom severity change from year 3 to year 6 after adjustment for other variables. Conclusions: ADHD in preschoolers is a relatively stable diagnosis over a 6-year period. The course is generally chronic, with high symptom severity and impairment, in very young children with moderate-to-severe ADHD, despite treatment with medication. Development of more effective ADHD intervention strategies is needed for this age group. (Contains 2 figures and 7 tables.)