This study assessed the psychometric properties of the PDD Behavior Inventory (PDDBI) as a diagnostic instrument forautism spectrum disorders. Individually matched samples of children and adolescents referred for evaluation of possibleautism spectrum disorders (ASDs) were rated by their parents on the instrument. The sample consisted of 84 children and adolescents between the ages of 37 and 141 months (42 each in an ASD group and a non-ASD group). Optimal sensitivity and specificity were achieved using a cutoff score of 45 on the Autism Composite T-score. Independent samples T-tests indicated that groups differed significantly on the Autism Composite T-score and on one of the 14 subscale/composite scores. Analyses were also conducted on individually matched IQ-based subgroups of participants (one group consisting of children and adolescents with nonverbal IQs less than or equal to 70, and the other consisting of children and adolescents with nonverbal IQ s greater than 70). The cutoff score of 32 that was recently proposed by the developers of the instrument was also evaluated in the entire sample and in the IQ-based subgroups. Results indicated that the instrument performed relatively poorly overall, with slightly better results obtained when used in the subsample of children and adolescents with nonverbal IQs less than or equal to 70.