Objective: The objective was to determine the differential validity of a cognitive disengagement syndrome-only (CDS-only) group from ADHD-inattentive presentation-only (IN-only), ADHD-hyperactive-impulsive presentation-only (HI-only), and ADHD-combined presentation (C-only) groups within childhood (ages 5-10) and adolescence (ages 11-16). Methods: Parents of a nationally representative sample of 5,525 Spanish youth (ages 5-16, 56.1% boys) completed measures of CDS, ADHD-inattention (IN), and ADHD-hyperactivity/impulsivity (HI) and other measures. Scores greater/less than the top 5% on CDS, IN, and HI were used to create: (1) control, (2) CDS-only, (3) ADHD-IN-only, (4) ADHD-HI-only, (5) ADHD-C-only, (6) CDS + ADHD-IN, (7) CDS + ADHD-HI, and (8) CDS + ADHD-C groups within childhood and adolescence. Results: Within childhood, the CDS-only group had higher scores than the three ADHD presentations on anxiety, depression, somatization, daytime sleep-related impairment, nighttime sleep disturbance, social impairment, and peer withdrawal, whereas CDS-only, ADHD-IN-only, and ADHD-HI-only groups did not differ on oppositional defiant disorder (ODD) and academic impairment (ADHD-C-only higher). Within adolescence, the CDS-only group again had higher scores than the three ADHD presentations on somatization and daytime sleep-related impairment but was now lower than the three ADHD presentations on ODD as well as lower on academic impairment than the ADHD-IN-only and ADHD-C-only groups. Within adolescence, the CDS-only group and the three ADHD presentations did not differ on depression, social impairment, or peer withdrawal. Conclusion: The CDS-only group had strong differential validity from ADHD-IN-only, ADHD-HI-only, and ADHD-C-only groups within childhood with less striking differences in adolescence. In addition to more studies with adolescents, etiological models with multi-informant longitudinal data are needed to better understand the differences in CDS and ADHD dimensions and their changes across development.