The effectiveness of methylphenidate (MPH), 0.3 and 0.5 mg/kg 2x daily, in the treatment of 31 children, aged 4 to 6 years, with attention-deficit hyperactivity disorder (ADHD) was investigated by a double-blind, placebo-controlled study at the University of Ottawa, Canada. Comorbid oppositional defiant disorder and conduct disorder were present in 84% and 19%, respectively. While on MPH compared to placebo, significant improvements were obtained on a cognitive measure (number of correct responses on Gordon Vigilance Task), the parent ratings of the child’s behavior, and tasks measuring ability to stick with a paper-and-pencil assignment. Parents’ ratings were most sensitive. A positive response to MPH on at least one of the measures of attention was obtained in 90% of patients. Improved performance during MPH was also noted in measures of impulsivity-hyperactivity and conduct. No changes were observed in the childs’ compliance with parental everyday requests. Side effects, stomachaches, headaches, anxiety, and sadness, increased in frequency and severity with the higher dosage of MPH. 
COMMENT. Methylphenidate may improve attention and parent-rated behavior of preschool children with ADHD, and comorbidity with oppositional defiant disorder is not a contraindication to its use in younger children. The positive effects of MPH in preschoolers parallels the benefits noted in older children, and side effects are similar and dose related.