Background: The supports paradigm has shifted focus from assessing competence and deficits among people with disabilities to identifying supports needed to live meaningful and productive lives in inclusive settings. Consequently, a rigorous and robust system is required that is capable of accurately determining the type and intensity of support needed and of allocating resources accordingly. The aim of the present study was to develop such a system to identify and classify support needs of people with disabilities based on the conceptual framework of the "International Classification of Functioning, Disability and Health" (ICF) [WHO, "The International Classification of Functioning, Disability and Health" (ICF), Author, Geneva, 2001], and the supports concept ["Mental Retardation: Definition, Classification and Systems of Support," 9th edn (1992), 10th edn (2002), American Association on Mental Retardation, Washington, DC). Method: A total of 1012 individuals with disabilities who were supported by accommodation and day programme organizations across the eastern states of Australia were assessed. The instrument was used in a team setting involving the person, their family and friends and staff as appropriate. "Version 1" was administered with 595 people with disability. This version was refined according to qualitative and quantitative analyses. Another 342 individuals were assessed using "Version 2," resulting in a combined data set for 936 individuals. "Version 3" was then trialled with a further 76 individuals with disabilities. Results: Ten domain scales in Health and Well Being (HWB) and Activities and Participation (A&P) were explored and refined. The scales effectively discriminated a range of intensities of support for people with various disabilities, with the highest support needs generally recorded by individuals with multiple disabilities who were ageing. The instrument can be used to develop a profile of needed supports across the domain scales. These measure current and predicted support needs, and contribute to future planning. The team approach proved beneficial in this regard. Conclusions: The "I-CAN" is a useful instrument for effectively assessing the support needs of people with a disability using a person centred approach. It is effective in identifying support needs across health and well-being areas, and activities of daily living. [For part II, see EJ841517.]