We examined the phenotypic expression of attention-deficit/hyperactivity disorder (ADHD) across cultures by assessing the factor/latent class structure of its core symptoms. We conducted a systematic review of the literature published from January 1987 to November 2008 using Medline and PsycINFO. We systematically reviewed 2,511 article abstracts, and 48 of these abstracts were included in this review. Research with school-age children from 15 countries including different samples, informants, and rating instruments supported a two-factor ADHD model consisting of inattention and combined hyperactivity and impulsivity. This model was not supported for preschool children. Cross-cultural equivalence for the two-factor model was suggested in some studies. Latent class analyses using parental data and more recent approaches like factor mixture modeling are generally consistent with factor analyses. These findings argue in favor of a cross-cultural validity of the syndrome. The implications of these findings for further research and classificatory systems in mental health like the "Diagnostic and Statistical Manual of Mental Disorders" (5th ed.) are discussed. (Contains 1 table.)