We evaluated the effect of OROS methylphenidate (MPH) on sleep quality and architecture in children with attention-deficit/hyperactivity disorder (ADHD) using both a parental sleep questionnaire and polysomnography. Twenty-four ADHD children who had no comorbid psychiatric or sleep disorders except for oppositional defiant disorder completed the 6-week, prospective, open-label, flexible-dose trial with OROS MPH (Concerta) monotherapy. After OROS MPH administration, the polysomnography data indicated that the percentage of stage 2 sleep was increased (P=0.024) and the Number of Awakenings was decreased (P=0.047). Relative to baseline, Parasomnias of the Children's Sleep Habits Questionnaire were decreased (P=0.033). Sleep Onset Latency was not changed during the treatment in general, but was increased in six children with subjective sleep difficulties (F(1)=5.832, P=0.025, eta(2)(p)=0.226). Bedtime Resistance and Sleep Onset Delay in Children's Sleep Habits Questionnaire were also increased during the treatment with OROS MPH only in individuals with sleep complaints (F1=5.001, P=0.036, eta(2)(p)=0.185; F(1)=7.237, P=0.013, eta(2)(p)=0.248). These results suggest that OROS MPH in open-label treatment does not seem to impair sleep and may even improve some aspects of sleep.