Prevalence rates of hyperactivity in Italy using commonly accepted cutoff scores in the United States were 20% for boys and 3% for girls; the combined prevalence rate was 12%. Italian children exhibited rates of hyperactive behaviors similar to those of children in New Zealand, Spain, and portions of the United States. Nevertheless, rates of hyperactivity are influenced by ethnic and cultural factors, and differences in prevalence rates may be expected in many situations. Factor structures of the Conners' Teacher Rating Scale for boys and girls were different. The need for separate factor analyses for males and females is stressed; hyperactivity and conduct problems, in fact, were separate factors for girls whereas such behaviors were subsumed under one factor for boys. There were also important similarities and differences with the factor structures for boys and girls in Italy and Spain. Methodological issues such as complete specification of the sample and assuring representativeness of samples are discussed.