Older adults may undergo changes in occupational participation patterns due to the process of aging, to institutionalization, or to illness. Measuring occupational participation of older adults using a client-centered approach has become a professional mandate for occupational therapists. The Activity Card Sort was developed to measure the level of client activity participation in instrumental, socio-cultural, and leisure activities. With the authors' permission, a valid method of translation and cultural adaptation of the ACS was developed based on a sample of older Puerto Rican adults and occupations relevant to this population derived from a previous study. The final version used in this analysis included 82 picture cards of older adults participating in typical Puerto Rican occupations which clients were to sort into five categories. The purpose of this study was to determine the validity and reliability of the translated and culturally-adapted Puerto Rican Spanish version of the Activity Card Sort (PR-ACS) for older Spanish-speaking adults living in Puerto Rico. This study included 146 participants, two groups of 106 community-living, healthy older adults aged 60 or older and 40 adults with multiple sclerosis (MS) aged 50 or older. The study explored evidence of validity by examining the relationship of the PR-ACS current activity scores with other variables. Reliability evidence included test-retest and internal consistency. Results showed that the PR-ACS was able to discriminate between clients with different levels of functioning (t = 6.86; p = .00), and was positively associated with the Puerto Rican Version of the RAND 36-Short Form Health Survey (r = 0.66; p = .00). Good test-retest reliability (r = 0.82) and high internal consistency of the total scores of the combined sample (r = 0.91) and the sample of healthy older adults (.91) were demonstrated, as well as good internal consistency of these scores for the sample of individuals with MS (.77). The findings suggest the PR-ACS is a reliable and valid instrument to use with the Puerto Rican elderly population.