The purpose of this study was to test whether low serum vitamin B-12 levels are associated with more rapid bone loss in elderly women. We archived sera and measured calcaneal bone mineral density (BMD) in community-dwelling white women, aged 65 yr and over, who participated in the Study of Osteoporotic Fractures. BMD of the hip and subregions was measured 2 yr later. Repeat measurements of calcaneal and hip BMD were obtained after 5.9 and 3.5 yr of follow-up, respectively. Serum vitamin B-12 assays were performed in 83 randomly selected participants with initial and repeat measurements of BMD who were not taking estrogen replacement therapy at baseline. After adjusting for age, weight, and clinic site, women with vitamin B-12 levels at or below 280 pg/ml (207.2 pmol/liter; lowest quintile) experienced an annual change of -1.6% (95% confidence interval, -2.4% to -0.8%) in total hip BMD, compared with -0.2% (-0.5% to 0.2%) in women with levels above 280 pg/ml (P = 0.003). Results were similar when subregions of the hip were analyzed separately. Serum vitamin B-12 levels were not significantly associated with calcaneal bone loss. We conclude that low serum vitamin B-12 levels are associated with increased rates of hip, but not calcaneal, bone loss in older women.