Examines reliability of test scores for the Pervasive Developmental Disorders Rating Scale (PDDRS), a screening instrument used in the assessment of autistic disorder. The results indicated that coefficient alpha for the PDDRS Total Score was adequate for screening purposes for both age groups studied. The results of the test-retest study also suggested that PDDRS had adequate test-retest reliability. (Contains 22 references and 3 tables.) (GCP)