Studies highlight that the functional deficits in different areas of a subject's life are an important characteristic that define adult attention-deficit/hyperactivity disorder (ADHD). On the other hand, in the scientific literature, there are no evaluation instruments with psychometric studies concerning their reliability and validity for this variable in adults with ADHD. The aim of the present study is to evaluate the psychometric properties of the Functioning Assessment Short Test (FAST), regarding its reliability and validity, as a measure of adult ADHD functioning. A case-control study was carried out in a sample of 152 adult subjects (88 with ADHD diagnosis and 64 healthy controls). The psychometric properties of the instrument were analyzed regarding feasibility, internal consistency, concurrent validity, discriminant validity (ADHD vs. controls) and factor analysis. For the total scale, Cronbach's alpha was of 0.83, and strong values in the measures of its discriminant capacity were obtained, AUC ROC = 0.98, IC (0.96-0.99). The test is reliable as the internal consistency was high. Significant differences are observed in the correlation between domains, between healthy subjects and subjects with ADHD. ADHD subjects showed impairments in all areas of their life, especially in the cognitive functioning domain, followed by the autonomy, occupational functioning and interpersonal relationships domains. The FAST is an easily administered short interview and has good psychometric properties, in terms of reliability and validity, as a measure of the functional level in adults with ADHD. The study also showed that subjects with adult ADHD may be functionally impaired.