Objective: Methylphenidate is a leading first-line treatment for ADHD (AD/HD). This stimulant has long been suspected to adversely affect sleeping patterns of treated individuals, especially children. There are few studies on the effects of recently developed longer-acting methylphenidate treatments, such as once-daily oral or transdermal formulations, on sleep. Method: The authors examined eight indices of sleep behavior among children treated with either of these two methylphenidate preparations or placebo in a randomized, double-blind, multicenter, parallel-group study. Results: The main predictor of sleep problems was baseline numbers or severity of preexisting sleep problems, whereas the different treatments and placebo varied little in their propensity to elicit such problems. There was no significant relationship between dosage and severity or frequency of sleep problems. Conclusion: The authors found little evidence that methylphenidate treatment (at least in sustained-release forms) was a significant cause of sleep problems in treated children who were carefully titrated to an optimal dose. (Contains 1 table and 4 figures.)