Background: Autism spectrum disorder often co-occurs with other psychiatric disorders. Although a high prevalence of autistic-like traits/symptoms has been identified in the pediatric psychiatric population of normal intelligence, there are no reports from adult psychiatric population. Subjects and methods: The subjects were78 adults of normal intelligence between 18 and 60 years of age (bipolar disorder, n=26; schizophrenia, n=26; healthy controls, n=26). Autistic-like traits/symptoms were measured using the Autism Spectrum Quotient (AQ) for adults. Symptom severity was measured using the Positive and Negative Symptoms Scale, the Hamilton Depression Rating Scale, and/or the Young Mania Rating Scale. Results: schizophrenic patients had higher level of autistic traits than bipolar patients and healthy control. On the other hand, bipolar patients had similar level of autistic traits as that of healthy control. Conclusion: These findings suggest the importance of evaluating autistic-like traits/symptoms underlying adult-onset psychiatric disorders for the best-suited treatment. Further studies with a prospective design and larger samples are needed.