Serum levels of vitamin D3, 25(OH)D3, and vitamin D binding protein (DBP) were measured in a group of elderly subjects (71-96 years) and a group of young adults (22-40 years). The mean serum levels of vitamin D3, 25(OH)D3 and DBP were significantly lower in the elderly subjects compared to the young adults. The mean molar ratio of 25(OH)D3 to vitamin D3 was significantly higher in the elderly compared to the young adults. A parabolic relationship was found between the serum levels of 25(OH)D3 and vitamin D3 in both groups. This relationship gives a higher ratio of the serum levels of 25(OH)D3 to vitamin D3 in the lower part of the vitamin D3 range. Our data do not indicate impaired hepatic 25-hydroxylation of vitamin D3 in the elderly.