This study describes the distribution and determinants of plasma 25-hydroxyvitamin D [25(OH)D] concentrations and risk factors for low 25(OH)D (< or = 37.5 nmol/L) in 290 men and 469 women aged 67-95 y who were in the Framingham Heart Study cohort. Mean (+/-SD) 25(OH)D concentrations were 82 +/- 29 nmol/L in men and 71 +/- 29 nmol/L in women. 25(OH)D was low in 6.2% of men and 14.5% of women. 25(OH)D concentrations were strongly associated with season of examination, inversely associated with time spent indoors and body mass index, and positively associated with dietary vitamin D intake. In women, concentrations were also inversely associated with age and positively associated with supplemental vitamin D intake and residence for > or = 3 mo/y in Florida, California, or Arizona, and in men were positively associated with serum creatinine concentrations. Similar amounts of variance in 25(OH)D concentrations were explained by vitamin D intake and sunlight exposure, the former being more important in women and the latter in men. None of the known or suspected determinants of vitamin D status could explain the lower 25(OH)D concentrations in women, but the sex difference was not seen for individuals examined during the winter. Results from this population-based sample of elderly individuals suggest that inadequate vitamin D status is an important public health problem, which could be readily addressed by adequate vitamin D intake or sunlight exposure.