The serum vitamin D2 and vitamin D3 metabolite concentrations and intestinal absorption of vitamin D2 were determined in healthy ambulatory and chronically institutionalized elderly subjects with normal renal function. The 25-hydroxyvitamin D (25OHD) concentrations were normal in all subjects (range, 8-43 ng/ml), although institutionalized subjects had a significantly lower mean value [19.2 +/- 2 (+/- SEM) ng/ml; P less than 0.01] compared with ambulatory subjects (25.3 +/- 2 ng/ml). All but one ambulatory subject had 25OHD3 as the major circulating form, whereas 25OHD2 was the major circulating metabolite in one third of the institutionalized subjects. The mean 1,25-dihydroxyvitamin D [1,25-(OH)2D] concentration in both groups was normal, but nine subjects had levels at or below the lower limit of normal despite normal 25OHD concentrations. Separate assay of 1,25-(OH)2D2 and 1,25(OH)2D3 revealed proportional distributions similar to those for 25OHD2 and 25OHD3. To study the effect of age on the intestinal absorption of vitamin D, we compared serum vitamin D2 concentrations after oral administration of 50,000 IU vitamin D2 in both healthy vitamin D-sufficient elderly subjects and young adults. We found no evidence of malabsorption of vitamin D in the elderly subjects. In summary, elderly subjects in New York, whether institutionalized or not, have normal serum 25OHD concentrations. However, while most elderly subjects have normal serum 1,25-(OH)2D levels, a significant proportion fail to produce normal concentrations of 1,25-(OH)2D, possibly due to age-related disturbances in renal synthesis of the hormone.