Background: There is an urgent need for developing reliable, valid and practical instruments that assess and classify the support needed by persons with disability to function in their chosen living, working and social environments. I-CAN is an instrument that addresses the frequency and level of support needed (not individual skills or deficits) for each individual with a disability. Method: Studies were conducted to assess the test-the relationship between the I-CAN domain scales and the Inventory for Client and Agency Planning (ICAP) (Bruininks et al. 1986) and the Quality of Life Questionnaire (QOL-Q) (Schalock & Keith 1993). Predictive validity studies were undertaken using day- and night-time support hours. Regression analyses were run using these measures with I-CAN domain scales. Two independent studies were also conducted to ascertain the practical utility of the instrument. Results: The I-CAN instrument demonstrated excellent inter-rater and test-retest reliability in the Activities and Participation domains. Low-to-moderate test-retest results in Physical Health, Mental Emotional Health and Behaviour domains were tracked to actual change in support needs in these areas. Validity proved acceptable. The relationships between I-CAN domain scales and adaptive behaviour were mixed but in the expected direction. Low-to-moderate correlation coefficients were evident between the I-CAN scales and the QOL-Q Total, but greater support needed in certain domains was associated with less empowerment and independence, and less community integration and social belonging. Attempts to explain current support hours against the I-CAN scales were disappointing and suggest that a number of other factors apart from individual support need to play a significant role. There was general satisfaction with the assessment process from stakeholders and participant groups. Conclusions: I-CAN is a reliable, valid and user-friendly instrument for assessing the support needs of people with disabilities. It uses a process that involves the persons with disability, their family and friends and staff as appropriate. It is also apparent that the current provision of paid support hours by agencies is a complex phenomenon that is not based solely on individual support needs. Further research is warranted on the influence of the environment and the perceptions of need for support based on negotiable and non-negotiable support needs. [For part I, see EJ843513.]