The study found that 11 raters who scored the CYBOCS-PDD showed excellent reliability (ICC=0.97). The mean CYBOCS score was 14.4 (plus or minus 3.86) with excellent internal consistency (alpha=.85). Correlations with other measures of repetitive behavior ranged from r=0.11 to r=0.28 and were similar to correlations with measures of irritability (r=0.24) and hyperactivity (r=0.25). Children with higher scores on the CYBOCS-PDD had higher levels of maladaptive behaviors and lower adaptive functioning. The study shows that the five-item CYBOCS-PDD is reliable, distinct from other measures of repetitive behavior, and sensitive to change. The modified scale (CYBOCS-PDD), which contains only the five compulsion severity items, was administered to 172 medication-free children (mean age 8.2 years) with PDD (autistic disorder, n=152; Asperger's disorder, n=6; and PDD not otherwise specified, n=14). All of the children were participating in RUPP clinical trials. Reliability was assessed by intraclass correlation coefficient (ICC) and internal consistency by Cronbach's alpha coefficient. Correlations with ratings of repetitive behavior and disruptive behavior were examined for validity. 3 tables, 1 figure, and 29 references