OBJECTIVE: The aim of this study was to describe the clinical presentation of preschoolers diagnosed with moderate to severe attention-deficit/hyperactivity disorder (ADHD) recruited for the multisite Preschool ADHD Treatment Study (PATS). The diagnosis and evaluation process will also be described. METHOD: A comprehensive multidimensional, multi-informant assessment protocol was implemented including the semistructured PATS Diagnostic Interview. Parent and teacher-report measures were used to supplement information from interviews. Consensus agreement by a cross-site panel on each participant's diagnoses was required. Analyses were conducted to describe the sample and to test associations between ADHD severity and demographic and clinical variables. RESULTS: The assessment protocol identified 303 preschoolers (3-5.5 years) with moderate to severe ADHD Hyperactive/Impulsive or Combined type. The majority of participants (n = 211, 69.6%) experienced co-morbid disorders, with oppositional defiant disorder, communication disorders, and anxiety disorders being the most common. Participants with co-morbid communication disorders were found to be more anxious and depressed. ADHD severity was found to correlate with more internalizing difficulties and lower functioning. Although boys and girls had similar symptom presentations, younger children had significantly higher ADHD severity. CONCLUSIONS: Preschoolers with moderate to severe ADHD experience high co-morbidity and impairment, which have implications for both assessment and treatment.