This six-site follow-up application requests funding for a five year study of the safety of methylphenidate (MPH) in children who had been originally recruited and randomized into the preschool Attention-Deficit / Hyperactivity Disorder (ADHD) treatment study (PATS). We propose to continue our multisite collaboration in order to conduct a systematic follow-up study of the children who were diagnosed with ADHD in preschool years (ages 3-5 years) and participated in the original Preschoolers with ADHD Treatment Study (PATS) to determine the safety and efficacy of methylphenidate. We estimate that of the about 300 children meeting PATS entry criteria, about 200 will be available for follow-up. The proposed follow-up study will assess these children as they turn 5-9 years of age. It will be based at the New York State Psychiatric Institute (NYSPI), Duke University, Johns Hopkins University (JHU), New York University (NYU), University of California at Los Angeles (UCLA), and University of California at Irvine (UCI). Aim 1 is to address whether early treatment with MPH is associated with emergence of any negative physical (decreased height and weight growth rates), cognitive, or behavioral (e.g., sleep disturbance, tics) effects. Aim 2 is to determine the continuity of dose-response relationships for methylphenidate as the child grows older, including its ability to decrease ADHD symptoms, decrease symptoms from comorbidities, and improve overall functioning. Aim 3 is to ascertain the extent to which the children's initial diagnostic and treatment status and treatment response impact on type of services received during the five-year follow-up period. Aim 4 is to determine the stability of the ADHD diagnosis and other comorbidities made for study entry at age 3-5 years across subsequent development [unreadable] [unreadable]