OBJECTIVES: To evaluate the validity and reliability of a standardized approach for assessing life-space mobility (the University of Alabama at Birmingham Study of Aging Life-Space Assessment (LSA)) and its ability to detect changes in life-space over time in community-dwelling older adults. DESIGN: Prospective, observational cohort study. SETTING: Five counties (three rural and two urban) in central Alabama. PARTICIPANTS: Community-dwelling Medicare beneficiaries (N = 306; 46% male, 43% African American) who completed in-home baseline interviews and 2-week and 6-month telephone follow-up interviews. MEASUREMENTS: The LSA assessed the range, independence, and frequency of movement over the 4 weeks preceding assessments. Correlations between the baseline LSA and measures of physical and mental health (physical performance, activities of daily living, instrumental activities of daily living, a global measure of health (the short form-12 question survey), the Geriatric Depression Scale, and comorbidities) established validity. Follow-up LSA scores established short-term test-retest reliability and the ability of the LSA to detect change. RESULTS: For all LSA scoring methods, baseline and 2-week follow-up LSA correlations were greater than 0.86 (95% confidence interval= 0.82-0.97). Highest correlations with measures of physical performance and function were noted for the LSA scoring method considering all attributes of mobility. The LSA showed both increases and decreases at 6 months. DISCUSSION: Life-space correlated with observed physical performance and self-reported function. It was stable over a 2-week period yet showed changes at 6 months.