The Index of ADL was developed to study results of treatment and prognosis in the elderly and chronically ill. Grades of the Index summarize over-all performance in bathing, dressing, going to toilet, transferring, continence, and feeding. More than 2,000 evaluations of 1,001 individuals demonstrated use of the Index as a survey instrument, as an objective guide to the course of chronic illness, as a tool for studying the aging process, and as an aid in rehabilitation teaching. Of theoretical interest is the observation that the order of recovery of Index functions in disabled patients is remarkably similar to the order of development of primary functions in children. This parallelism, and similarity to the behavior of primitive peoples, suggests that the Index is based on primary biological and psychosocial function, reflecting the adequacy of organized neurological and locomotor response.