Because of changes that occur with aging, older people with any other risk factors for vitamin D deficiency are likely to have inadequate stores of this vitamin. The consequences of vitamin D deficiency are likely to be losses in bone, strength, and function and the development of pain. Many questions remain regarding screening, prevention, and treatment of vitamin D deficiency. Supplementation may be unnecessary in most healthy, ambulatory seniors. Excessive supplementation in this group may lead to vitamin D toxicity. There does seem to be a role for supplementation in homebound older people who will not get adequate vitamin D from sunlight exposure. This population is at particular risk of developing vitamin D deficiency. Issues such as inadequate diet, physiologic changes with aging, polypharmacy, and diseases that interfere with vitamin D metabolism contribute to this risk. In such circumstances, a recommendation of 800 IU per day is reasonable. An alternative to daily dosing is a single oral dose of 100,000 IU of vitamin D (ergocalciferol or cholecalciferol) every 3 to 6 months. A simple maneuver is for geriatricians, who see many chronically ill patients with low vitamin D stores (who are likely to be seen in the office every 3 to 6 months), to administer vitamin D during the office visits. These dosing schedules have not been associated with toxicity and can be considered safe in homebound (sunlight-deprived) older adults.(ABSTRACT TRUNCATED AT 250 WORDS)