The attention-deficit/hyperactivity disorder (ADHD) is a common psychiatric disorder that interferes with the typical development and both learning and motor functioning in a child's life. Most of the children with ADHD present also sleep problems like difficulties in falling asleep and maintaining sleep. Sleep spindles are characteristic waves of sleep stage 2 in humans and are characterized by a fusiform morphology. In the last years, the empirical evidence indicates that spindles are associated with cognitive faculties and intelligence as well as with several disease states. On the other hand, power spectral analysis of EEG represents a powerful noninvasive tool for examining cerebral behavior. The aim of this study is to evaluate the differences between ADHD and healthy children of the power spectral values in delta, theta, alpha, beta and gamma bands, before, during and after sleep spindles. Our results show significant differences concentrated in the period immediately after spindle epochs, in the left hemisphere of the brain, in almost all bands, with greater values in control than in ADHD children.