In the field of intellectual disability (ID) generic quality of life (G‐QoL) has been reported to be much more useful than health‐related QoL. Despite this, Italian‐language tools with validated assessment of G‐QoL in people with ID are lacking. The authors undertook a study to test the psychometric properties of the BASIQ, an Italian adaptation of the Quality of Life Instrument Package developed at the Centre for Health Promotion, University of Toronto, Canada, in the early 1990s. This instrument package integrates qualitative and quantitative measures and subjective and objective approaches in assessing nine domains of QoL using a direct interview (DI), a proxy questionnaire (PQ), and an external assessor questionnaire (EAQ). Using BASIQ, the study assessed the QoL of a sample of 281 adults with ID (47 DI by 11 raters, 260 PQ by 23 proxies, and 69 EAQ by 5 physicians) drawn randomly from a three services populations in central Italy. The adults, 69% men and 31% women, ranged in age from 16 to 86 (X = 41.57); most (67.6%) functioned in the mild and moderate ranges of ID. QoL assessments were carried out in Italian, and were completed by specially trained disability professionals. The instrument had good internal consistency (Cronbach's alpha 0.990) and satisfactory inter‐rater reliability (Cohen's K > 0.7). For each of the DI and the PQ, all nine domains correlated positively (p < .05), and this was also the case for most domains of the EAQ. Post hoc analyses found no differences in QoL on any of the three scales for gender or co‐occurrence of mental or physical problems, but revealed some differences for level of disability. Overall, the authors present evidence for considering the BASIQ to be a valid instrument to assess QoL and put it forward as a useful tool to use in practice and policy development.