Although Attention Deficit/Hyperactivity Disorder (ADHD) has increasingly been studied in preschool-aged children, relatively few studies have provided a comprehensive evaluation of the factor structure and patterns of developmental changes in parent-reported ADHD symptomatology across the early childhood period. This study used confirmatory factor analyses to test for longitudinal measurement invariance of ADHD symptoms and semi-parametric finite mixture models to identify prototypic patterns of developmental changes in ADHD symptomatology from 3 to 5 years of age. Participants were 1155 children and their parents who participated in a prospective longitudinal study involving a representative sample of children who resided in six non-metropolitan counties in the United States. Results indicated that (1) ADHD symptomatology was best represented by a single latent factor that exhibited partial measurement invariance from 3 to 5 years of age, (2) 8.5 % of children exhibited sustained high levels of ADHD symptoms from age 3-5 years, and (3) a variety of risk factors differentiated children with sustained high from those with sustained low levels of ADHD, relatively few (most notably caregiver education) were able to differentiate children with sustained high levels of ADHD symptoms from all other groups. Children who exhibit persistent ADHD symptomatology across the early childhood period may define a clinically important group for etiologic research and/or early intervention efforts.