In a longitudinal study, two boys in the Outpatient Psychiatric Clinic at Tokyo University were found to exhibit Tourette's disorder in addition to the original diagnoses of infantile autism. This paper addresses problems of applying the diagnostic criteria of DSM-III in terms of voluntary tic suppression in diagnosing patients with both disorders. Differences between motor tics and stereotyped movements in patients with both infantile autism and Tourette's disorder have been clearly distinguished. This may enable us to identify more autistic individuals with Tourette's disorder by focusing on these differences. In contrast to Burd et al.'s findings and implications, these two boys have not manifested spurts in language and social relationships nor have their conditions significantly improved, despite the development of Tourette's disorder.