Subthreshold autism spectrum in bipolar disorder: Prevalence and clinical correlates. BACKGROUND:  While few previous studies highlighted a higher prevalence of autistic traits among adults with Bipolar Disorder (BD), little is known about their clinical significance in this population. METHOD:  143 subjects with BD were enrolled at the adult psychiatric inpatient clinic of the University of Pisa. Assessments included the SCID-5, the MOODS-SR, the AQ and the AdAS Spectrum. RESULTS:  42.7% of the sample scored positively for significant levels of autistic traits. Subjects with high autistic traits showed a greater likelihood of a very early onset of BD, greater length of current in-hospital stay, significantly higher rates of anxiety disorders and lower rates of substance use disorders compared to patients with low autistic traits. They also show significantly greater depressive symptoms and suicidality across the lifetime. Suicidality was associated with the altered responsiveness to sensory input and inversely related to adherence to routine and inflexibility. CONCLUSION:  The study is a first exploration of the clinical significance of autistic traits among BD patients. Our results highlight the clinical significance of autistic traits in patients with BD, supporting the usefulness of a dimensional approach to the autism spectrum.