The Swanson, Nolan, and Pelham scale version IV (SNAP-IV) is widely used to assess symptoms of attention deficit hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) in children and adolescents. Nevertheless, there is insufficient data to support its use in preschool children. The study had three goals: First, to test the factorial validity of the three correlated-factors model of ADHD and ODD items of the SNAP-IV. Second, to investigate the measurement invariance of the items over time (6-month longitudinal interval) and by sex. Third, to investigate the convergent validity and method-specific influences on ADHD/ODD assessments with respect to multiple raters (parents/teachers) of children’s symptoms. Participants were 618 preschool children (3.5–6 years) at baseline and 6-month follow-up. For model testing, we used confirmatory factor analysis for categorical observed variables. Method and trait effects were examined using the CT-C(M-1) model. The analyses showed partial measurement invariance over time and according to sex. Moreover, strong rater-specific effects were detected. The implication of the results for construct validation of the instrument and clinical assessment of ADHD and ODD traits are discussed.