The purpose of this study was to investigate the validity of the Irena Daily Activity (IDA) assessment which measures occupational performance in adults with developmental disabilities. The instrument's content validity was investigated by elucidating its general appropriateness, its clinical applicability and the feasibility of the measurement procedure. Data were collected from a multidisciplinary panel of clinical experts working in day activity centres. Each expert independently judged and reviewed the IDA instrument's content validity. The results show that the content validity index (CVI) values ranged from 0.8 to 1.0 for the instrument's general appropriateness and clinical applicability, and from 0.7 to 1.0 with respect to the significance of the IDA domains' clinical applicability. The applicability of the IDA items were judged as satisfactory (CVI > 80) with respect to their importance for planning treatment and intervention. The feasibility of the measurement procedure was estimated as satisfactory in relation to the items (CVI > 0.70) and satisfactory with respect to the time needed and material used, with CVI values ranging from 0.8 to 1.0. The results of this study support the use of the IDA as a feasible and time-efficient assessment that provides insights regarding the occupational performance of adults with developmental disabilities. A limitation of this study was that it could not illuminate all-important aspects of validity. In conclusion, the IDA has the potential to become a reliable and valid clinical assessment, but additional research on psychometric properties is still needed.