"Sluggish cognitive tempo" (SCT) is a construct hypothesized to describe a constellation of behaviors that includes daydreaming, lethargy, drowsiness, difficulty sustaining attention, and underactivity. Although the construct has been inconsistently defined, measures of SCT have shown associations with symptoms of attention-deficit/hyperactivity disorder (ADHD), particularly inattention. Thus, better characterization of SCT symptoms may help to better predict specific areas of functional difficulty in children with ADHD. The present study examined psychometric characteristics of a recently developed 14-item scale of SCT (Penny et al., Psychological Assessment 21:380-389, 2009), completed by teachers on children referred for outpatient neuropsychological assessment. Exploratory factor analysis identified three factors in the clinical sample: "Sleepy/Sluggish", "Slow/Daydreamy", and "Low Initiation/Persistence". Additionally, SCT symptoms, especially those loading on the "Sleepy/Sluggish" and "Slow/Daydreamy" factors, correlated more strongly with inattentive than with hyperactive/impulsive symptoms, while "Low Initiation/Persistence" symptoms added significant unique variance (over and above symptoms of inattention) to the predictions of impairment in academic progress.