Objective: To investigate the effect of melatonin treatment on sleep, behavior, cognition, and quality of life in children with attention-deficit/hyperactivity disorder (ADHD) and chronic sleep onset insomnia. Method: A total of 105 medication-free children, ages 6 to 12 years, with rigorously diagnosed ADHD and chronic sleep onset insomnia participated in a randomized, double-blind, placebo-controlled trial using 3 or 6 mg melatonin (depending on body weight), or placebo for 4 weeks. Primary outcome parameters were actigraphy-derived sleep onset, total time asleep, and salivary dim light melatonin onset. Results: Sleep onset advanced by 26.9 plus or minus 47.8 minutes with melatonin and delayed by 10.5 plus or minus 37.4 minutes with placebo (p less than 0.0001). There was an advance in dim light melatonin onset of 44.4 plus or minus 67.9 minutes in melatonin and a delay of 12.8 plus or minus 60.0 minutes in placebo (less than 0.0001). Total time asleep increased with melatonin (19.8 plus or minus 61.9 minutes) as compared to placebo (-13.6 plus or minus 50.6 minutes; p = 0.01). There was no significant effect on behavior, cognition, and quality of life, and significant adverse events did not occur. Conclusion: Melatonin advanced circadian rhythms of sleep-wake and endogenous melatonin and enhanced total time asleep in children with ADHD and chronic sleep onset insomnia; however, no effect was found on problem behavior, cognitive performance, or quality of life. (Contains 1 figure and 3 tables.)