CONTEXT: Vitamin D deficiency is common among older adults, but the association between 25-hydroxyvitamin D [25(OH)D] levels and rates of bone loss is uncertain. OBJECTIVE: Our aim was to test the hypothesis that lower 25(OH)D levels are associated with higher rates of hip bone loss in older men. DESIGN AND SETTING: We conducted a prospective cohort study in six U.S. centers. PARTICIPANTS: A total of 1279 community-dwelling men aged 65 yr or older with 25(OH)D levels (liquid chromatography-tandem mass spectroscopy) and hip bone mineral density (BMD) (dual-energy x-ray absorptiometry) at baseline and repeat hip BMD an average of 4.4 yr later participated in the study. MAIN OUTCOME MEASURE(S): We measured the annualized percentage rate of change in hip BMD. RESULTS: After adjustment for multiple potential confounders, the average rate of decline in total hip BMD was -0.59%/yr among men with 25(OH)D levels below 15.0 ng/ml, -0.54%/yr among men with 25(OH)D levels 15.0-19.9 ng/ml, -0.35%/yr among men with 25(OH)D levels 20.0-29.9 ng/ml, and -0.37%/yr among men with 25(OH)D levels of at least 30 ng/ml (P trend = 0.008 for multivariable model). Evidence was strong to support an association among men aged 75 yr and older (P trend <0.001), but not among younger men (P trend = 0.55). Findings were similar when 25(OH)D level was expressed in quintiles and when BMD at hip subregions was substituted for total hip BMD. CONCLUSIONS: In this cohort of community-dwelling older men, men with 25(OH)D levels below 20 ng/ml had greater subsequent rates of hip bone loss, but rates of loss were similar among men with higher levels. These results lend support to the view that low 25(OH)D levels are detrimental to BMD in older men.