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Background: Although evidence is accumulating for a protective effect of late life physical activity on the risk of dementia, the findings are inconsistent, especially in men. We examined the association of late life physical activity and the modifying effect of physical function with future risk of dementia in a well-characterized cohort of elderly men participating in the Honolulu-Asia Aging Study (HAAS)., Methods: Physical activity by self-report and performance-based physical function was assessed in 2263 men aged 71-92 years without dementia at the baseline examination of the HAAS in 1991-1993. Follow-up for incident dementia occurred at repeat examinations conducted in 1994-1996 and 1997-1999. Analyses were based on Cox proportional hazards models with adjustment for potential confounders, including age, baseline cognitive function, education, and apolipoprotein E genotype., Results: There were 173 incident cases of dementia with a mean follow-up of 6.1 years. Although the incidence of dementia tended to decline with increasing physical activity and function, there was a significant interaction between the latter two factors on dementia risk (p =.022). For men with low physical function, high levels of physical activity were associated with half the risk of dementia versus men who were the least active (hazard ratio [HR], 0.50; 95% confidence interval [CI], 0.28-0.89), with a moderate level of physical activity also providing a protective effect (HR, 0.57; 95% CI, 0.32-0.99). Risk of dementia and Alzheimer's disease declined significantly with increasing physical activity. Findings persisted after age and risk factor adjustment. Similar associations were absent in men with moderate and high physical function., Conclusions: In elderly men with poor physical function, increasing general physical activity may potentially confer a protective effect or delay the onset for dementia.