The behavior of 60 children with Down syndrome (cDS), aged 4 to 21 years, was assessed by parent and teacher report using the Reiss psychopathology rating scale for dual diagnosis (mental disability plus mental illness). Standard forms contained 12 categories of abnormal behavior (e.g., Psychosis), each with five descriptive items (e.g., bizarre ideas) that could be scored as no problem, problem, and major problem (0, 1, or 2 points). Total scores for individual cDS (mean of two raters) ranged from 3 to 43 out of a potential 120 points, with a mean of 14.8 points and a standard deviation of 9.5. There were no significant differences by sex, age, or ethnicity. The highest aggregate scores among the 60 cDS were in the psychometric categories of Attention-Deficit (156 points), Anger/Self Control (154.5), and Psychosis (109) and for component items such as communication problems (55 points) under the Psychosis category; distracted (48.5) or disobedient (42.5) under the Attention-Deficit category; and temper tantrums (36.5), impulsive (32), or impatient (31.5) under the Anger/Self-Control category. Agreement between parent and teacher raters was over 75% for 38 of the 60 items and above 60% for an additional 15. Selection of cDS with scores over 10 points yielded 24 families who chose parental guidance and 14 who chose professional referral. The structure of the Reiss instrument was useful for cDS, distinguishing behaviors potentially related to medical problems from true psychopathology.