SPAID (Psychiatric Instrument for the Intellectually Disabled Adult) is the first Italian tool-package for carrying out psychiatric diagnosis in adults with Intellectual Disabilities (ID). It includes the "G" form, for general diagnostic orientation, and specific checklists for all groups of syndromes stated by the available classification systems. SPAID was established to provide an easy and quick tool for daily practice of the personnel working with ID. The present study was aimed at evaluating psychometric and psychodiagnostic characteristics of the SPAID-G and at supplying new data on the prevalence rate of psychiatric disorders in a multicentric Italian sample of people with ID living in different settings. The SPAID-G was randomly applied to 304 participants with ID attending residential facilities or assessment services across Italy. A part of the sample was also consecutively assessed through the use of DASH, PDD-MRS and by the clinical application of the DSM-IV TR criteria. The correlation between SPAID-G scores and those provided by other evaluation tools was over 60%. Additionally, the internal consistency and inter-rater reliability resulted to be good. Psychopathological symptoms were detected in approximately 40% of the sample. Respectively, autistic spectrum disorders, impulse control disorders, mood disorders, and dramatic personality disorders were the diagnostic orientations providing the most prevalent over-threshold scores. SPAID-G seems to be a valid diagnostic tool, quick and easy to use in psychiatric disorders assessment within the Italian population with ID. (Contains 2 tables and 1 figure.)