Previous studies suggested that melatonin improves sleep in insomniac patients with Angelman syndrome. To assess the efficacy of melatonin, a randomized placebo-controlled study was conducted in 8 children with Angelman syndrome with idiopathic chronic insomnia. After a 1-week baseline period, patients received, depending on age, either melatonin 5 or 2.5 mg, or placebo, followed by 4 weeks of open treatment. Parents recorded lights off time, sleep onset time, wake-up time, and epileptic seizures in a diary. Salivary melatonin levels were measured at baseline and the last evening of the fourth treatment week. Melatonin significantly advanced sleep onset by 28 minutes, decreased sleep latency by 32 minutes, increased total sleep time by 56 minutes, reduced the number of nights with wakes from 3.1 to 1.6 nights a week, and increased endogenous salivary melatonin levels. Parents were satisfied with these results. Indications that melatonin dose in Angelman syndrome patients should be low, are discussed.