Objective: To determine the impact of including sluggish cognitive tempo items on the factor and latent class structure of attention-deficit/hyperactivity disorder (ADHD) subtypes in boys and girls. Method: Parent report of two sluggish cognitive tempo items on a population-based sample of 1,430 female twins and 1,414 male twins were analyzed along with parent report of the 18 DSM-IV ADHD items using principal components analysis and latent class analysis. Results: The inclusion of the two sluggish cognitive tempo items resulted in distinct factor structures for boys and girls. For boys there were separate inattentive, hyperactive/impulsive, and sluggish factors. For girls there were separate inattentive/sluggish, hyperactive, and impulsive factors. In contrast, sluggish cognitive tempo items had minimal impact on the latent class structure of ADHD for both boys and girls. Conclusions: The inclusion of sluggish cognitive tempo items markedly changed ADHD symptom associations for boys and girls in a factor analytic framework. In contrast, latent class subtyping of ADHD shows limited impact of the inclusion of sluggish cognitive tempo items, emphasizing the very different assumptions about underlying continua of behavior rather than discrete classes that distinguish the two approaches. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(5):588-597. Key Words: attention-deficit/hyperactivity disorder, latent class analysis, factor analysis, sluggish cognitive tempo.