Prevalence of severe sleep problems and its association with other variables were investigated with 109 individuals who have Angelman syndrome. Severe settling problems, frequent night waking, and early waking were found in 2%, 37%, and 10% of the individuals, respectively. Sleep problems were persistent in this sample. No statistically significant associations were found between presence of a severe sleep problem and other variables (e.g., epilepsy, age, living environment, cause of genetic disorder, parents' and caregivers' coping strategies). Most parents reported adverse effects of their child's sleep problems on their own well-being. Implications for analysis and treatment of sleep problems in individuals with Angelman syndrome are discussed.