Background. The relationship between body mass index (BMI), weight loss, and changes in activities of daily living (ADL) function and mobility in older adults is not clear. We sought to study the relationship between BMI and weight loss on the rate of decline in ADL function and life-space mobility over a 4-year period among older African Americans and whites. Methods. The participants were 983 enrollees in the University of Alabama at Birmingham (UAB) Study of Aging, a longitudinal study of mobility among community-dwelling older adults stratified to achieve a balanced sample in terms of sex, race, and residence. Primary outcome measures were changes in ADL function and mobility assessed by the UAB Study of Aging Life-Space Assessment (LSA) which were measured every 6 months. Results. Relative to normal weight participants, those with BMI levels in the obese range did not show more rapid ADL functional decline, but a history of unintentional weight loss predicted more rapid decline. Relative to normal-weight participants, other BMI categories were not associated with more rapid decline in LSA scores. However, unintentional weight loss predicted more rapid declines in LSA. Intentional weight loss had no relation to ADL function or LSA decline. Conclusions. In this population of community-dwelling older African Americans and whites, neither BMI nor intentional weight loss had an association with rate of functional decline. Unintentional weight loss had a negative relation with rate of functional decline, regardless of baseline BMI. Whether this is causal remains to be determined. Copyright 2008 by The Gerontological Society of America.