Clinicians might minimize the prevalence of behavioral disorders among mentally retarded people. Decreased attention, hyperactivity, and impulsivity are frequently reported in children with Down syndrome, yet the exact prevalence of attention-deficit/hyperactivity disorder (ADHD) has not been clearly estimated in this population. The objective of this study was to estimate the prevalence of ADHD in children with Down syndrome and to emphasize the possible relationship between ADHD symptoms and the level of mental retardation and common medical comorbidity. In this study, the prevalence of ADHD among Down syndrome children was very high, reaching 43.9%. No significant correlation was found between ADHD symptoms and the level of mental retardation, but significant correlation was found with ophthalmologic problems. We conclude that children with Down syndrome are at increased risk for ADHD. When evaluating children with Down syndrome for attention deficits, psychiatric comorbidity as well as medical problems should be carefully taken into consideration.