The serum concentrations of 25-dihydroxyvitamin D, 1,25-dihydroxyvitamin D, 24,25-dihydroxyvitamin D, vitamin D-binding protein, PTH and calcitonin were measured in three groups of elderly Norwegian subjects (age 70-96 years): active elderly living at home, warded geriatric patients not supplemented with vitamin D, and warded geriatric patients supplemented with a daily dose of 400 IU vitamin D2. The results were compared with the concentrations of vitamin D metabolites found in a group of young and middle-aged adults (age 22-59 years). Decreased serum concentrations of 25-dihydroxyvitamin D3 were found in all groups of elderly compared with younger adults. Active elderly living at home had higher concentrations of 25-dihydroxyvitamin D3 than geriatric ward patients. Supplementation of geriatric ward patients with 400 IU vitamin D2 resulted in an increase in the median serum 25-dihydroxyvitamin D concentration by about 30 nmol/l. Decreased median concentration of 1,25-dihydroxyvitamin D was found in geriatric ward patients not supplemented with vitamin D, indicating that this group is at risk of vitamin D deficiency. The active elderly living at home and the warded geriatric patients receiving vitamin D supplementation had normal median concentrations of 1,25-dihydroxyvitamin D, indicating that nephrogenous synthesis of 1,25-dihydroxyvitamin D is not generally impaired in the elderly, and that a moderate vitamin D supplementation may correct low 1,25-dihydroxyvitamin D levels, owing to vitamin D deficiency. However, the serum concentrations of 1,25-dihydroxyvitamin D showed great individual variations. No significant differences were observed for vitamin D-binding protein, 'free-1,25-dihydroxyvitamin D, or PTH between the groups.(ABSTRACT TRUNCATED AT 250 WORDS)