Objective: The efficacy and safety of dexmethylphenidate extended release (d-MPH-ER) was compared to placebo in pediatric patients with attention-deficit/hyperactivity disorder (ADHD). Method: This multicenter, randomized, double-blind, placebo-controlled, parallel-group, two-phase study included 97 patients (ages 6-17 years) with "DSM-IV"-defined ADHD. The study was carried out between 2001 and 2003. After a 2-week evaluation phase, patients were randomized to d-MPH-ER or placebo for 7 weeks. Flexible d-MPH-ER dosing (30 mg/day) was permitted for 5 weeks, then patients remained on their optimal dose during the last 2 study weeks. The primary efficacy measure was change from baseline to final rating in Conners ADHD/DSM-IV Scale-Teacher version (CADS-T) total subscale score. Secondary efficacy variables included changes from baseline to final visit in CADS-T Inattentive and Hyperactive-Impulsive subscale scores, CADS-P "DSM-IV" total subscale score and Inattentive and Hyperactive-Impulsive subscale scores, Clinical Global Impressions-Improvement (CGI-I) and CGI-Severity (CGI-S) scale scores, and Child Health Questionnaire Parent Form 50 scores. Results: d-MPH-ER improved CADS-T total scores significantly compared with placebo (p less than 0.001), and 67.3% of d-MPH-ER patients were rated much improved or very much improved on CGI-I at final visit versus 13.3% of placebo patients (p less than 0.001). More patients taking d-MPH-ER (49.1%) than placebo (25.5%) spontaneously reported adverse events suspected as drug related. Conclusions: Once-daily d-MPH-ER was more effective than placebo in the treatment of ADHD in children and adolescents. (Contains 3 figures and 3 tables.)