BackgroundPatients with ‘underlying’ autism spectrum disorder (ASD) constitute a significant minority in adult out-patient psychiatry. Diagnoses of previously unrecognised ASD are increasing in adults. Characteristics of patients with autism within adult out-patient psychiatry have not been sufficiently explored, and there have not been any systematic comparisons of characteristics between patients with and those without autism within adult out-patient psychiatric populations.AimsTo examine psychiatrically relevant characteristics in autistic adult psychiatric out-patients, and to compare the characteristics with non-autistic adult psychiatric out-patients.MethodWe assessed 90 patients who were referred to a Swedish psychiatric out-patient clinic and screened for ASD during 2019–2020. Sixty-three patients met the DSM-5 criteria for ASD or ‘subthreshold’ ASD. The 27 who did not meet the criteria for ASD were used as a comparison group. Assessments were made with structured and well-validated instruments, including parent ratings of developmental history.ResultsNo differences were found between the groups regarding self-reported sociodemographic variables. The ASD group showed a higher number of co-occurring psychiatric disorders than the non-ASD group (t(88)= 5.17, 95% CI 1.29–2.91,d= 1.19). Functional level was lower in the ASD group (t(88)= −2.66, 95% CI −9.46 to −1.27,d= −0.73), and was predicted by the number of co-occurring psychiatric disorders.ConclusionsThe results underscore the need for thorough assessment of psychiatric disorders in autistic patients in adult psychiatric services. ASD should be considered as a possible ‘underlying’ condition in adult psychiatry, and there is no easy way of ruling out ASD in this population.