OBJECTIVE To estimate the prevalence of autism spectrum disorders (ASD) among adult psychiatric out-patients; to evaluate the efficacy of a new brief screening questionnaire (ASDASQ). METHODS 1323 adult psychiatric out-patients were screened by staff. Analysis of psychiatric records of patients (n = 66) scoring high on the ASDASQ yielded 31 patients with a suspected ASD. Twenty-two of these patients were clinically examined. Three psychometric aspects of the questionnaire were studied. RESULTS Seventeen patients were found by clinical examination to have an ASD. Since two patients scoring low on the ASDASQ were known to have an ASD, at least 19 patients in this population (1.4%) had a definite ASD. Seventeen of the ASD patients had been previously diagnosed with other psychiatric disorders, most frequently schizophrenia (n = 5). Of patients attending a treatment centre for severe psychiatric disabilities (n = 499), 3.2% had an ASD. The ASDASQ showed good reliability across and within raters. Internal consistency was excellent. CONCLUSIONS Adult psychiatric patients sometimes have undiagnosed autism spectrum disorders. The ASDASQ can be useful for screening.