The objective of this study was to evaluate the validity of a new parent rating scale of Sluggish Cognitive Tempo (SCT). SCT was defined with 10 symptom domains--daydreams; attention fluctuates; absentminded; loses train of thought; easily confused; seems drowsy; thinking is slow; slow-moving; low initiative; and easily bored, needs stimulation--with each domain represented by multiple examples. Mothers' and fathers' ratings of SCT, ADHD-IN, ADHD-HI, oppositional defiant disorder (ODD), and depression symptoms along with ratings of academic and social impairment were obtained for a sample of 802 Spanish first-grade children (54% boys). SCT Symptom Domains 4 to 8 showed substantial loadings on the SCT factor (i.e., convergent validity) and substantially higher loadings on the SCT factor than the ADHD-IN factor (i.e., discriminant validity). This 5-domain measure of SCT showed good interrater and test-retest reliability for a 6-week interval. Higher scores on the 5-domain measure of SCT predicted higher levels of academic and social impairment even after controlling for ADHD-IN and depression. In contrast, higher levels of SCT were not uniquely related (or uniquely negatively related) to ADHD-HI and ODD, whereas ADHD-IN and depression were uniquely positively related to ADHD-HI and ODD. The new measure of SCT more clearly establishes that SCT, ADHD-IN, and depression represent independent symptom dimensions, thus providing a measurement tool to help determine if SCT and ADHD-IN dimensions have unique biological correlates and if SCT and ADHD meet the criteria for different disorders.