Background: Older adults may be more prone to developing vitamin D deficiency than younger adults. Dietary requirements for vitamin D in older adults are based on limited evidence. Objective: The objective was to establish the dietary intake of vitamin D required to maintain serum 25-hydroxyvitamin D [25(OH)D] concentrations above various cutoffs between 25 and 80 nmol/L during wintertime, which accounted for the effect of summer sunshine exposure and diet. Design: A randomized, placebo-controlled, double-blind, 22-wk intervention was conducted in men and women aged >= 64 y (n = 225) at supplemental levels of 0, 5, 10, and 15 mu g vitamin D3/d from October 2007 to March 2008. Results: Clear dose-related increments (P 0.0001) in serum 25(OH)D were observed with increasing supplemental vitamin D-3 intakes. The slope of the relation between total vitamin D intake and serum 25(OH)D was 1.97 nmol . L-1 . mu g intake(-1). The vitamin D intake that maintained serum 25(OH)D concentrations >25 nmol/L in 97.5% of the sample was 8.6 mu g/d. Intakes were 7.9 and 11.4 mu g/d in those who reported a minimum of 15 min daily summer sunshine exposure or less, respectively. The intakes required to maintain serum 25(OH)D concentrations of >37.5, >50, and >80 nmol/L in 97.5% of the sample were 17.2, 24.7, and 38.7 mu g/d, respectively. Conclusion: To ensure that the vitamin D requirement is met by the vast majority (>97.5%) of adults aged >= 64 y during winter, between 7.9 and 42.8 mu g vitamin D/d is required, depending on summer sun exposure and the threshold of adequacy of 25(OH)D. This trial was registered at http://www.controlled-trials.com/ISRCTN20236112 as ISRCTN registration no. ISRCTN20236112. Am J Clin Nutr 2009;89:1366-74.