The Activity Card Sort (ACS) is a comprehensive instrumentfor assessing participation in occupational performance of instrumental, social-cultural, and leisure activities. The purpose of thestudy was to determine thereliability and validity of theACS within different adultand older adult groups. The study included 263 participants comprising 5 groups of 61 healthy adults, 61 healthy older adults, 40 spouses or caregivers of individuals with Alzheimer's, 45 individuals with multiple sclerosis, and 56 individuals 1 year after having a stroke. The ACS was adapted to the Israeli culture in a previous study with theauthor's permission. The final version that was used in this analysis included 88 picture cards of adults performing instrumental, social-cultural, and low and high physical leisure activities that the clients sort into five categories. The ACS provides a "retained activity level" score that is the percentage of activities in which a person is currently engaged divided by those with whom he or she was involved in the past. In addition, comparisons between all groups of current activity levels were analyzed. The ACS had high internal consistency (Cronbach alpha in each area) for instrumental activities of daily living (lAOL) and social-cultural activities (.82, .80), and lower for low and high physical leisure activities (.66, .61). A one-way analysis of variance (ANOVA) that compared groups of participants on total retained activitylevel and individual activityareas showed a significant group effect on allcomparisons (p<.OOO) thatsupport constructvalidity. Post hoc Scheffe tests revealed significant differences between mostgroups. The same results were foundfor current activitylevel. The results support thereliability andvalidity of the ACS and provide a basis for its clinical use.