OBJECTIVE Symptoms of sluggish cognitive tempo (SCT) are correlated with inattention and internalizing difficulties. The purpose of the present study was to determine whether symptoms of SCT reflect comorbid internalizing disorder with ADHD or a separate syndrome. METHODS Using a clinical sample of youth evaluated for behavioral and learning difficulties (N = 73), this study examined whether SCT remains associated with symptoms of ADHD after accounting for comorbid symptoms of anxiety and depression reported by children and parents. RESULTS SCT symptoms were correlated with inattention and parent reports of child depression, but not with parent-reported anxiety or child reports of internalizing problems. Inattention (in the absence of hyperactivity/impulsivity) remained uniquely associated with SCT even after accounting for internalizing problems. CONCLUSIONS The findings confirm SCT as a correlate of inattention and support its construct validity as separate from comorbid internalizing problems. Further research on the clinical utility of SCT is needed.