1. Radio-calcium absorption, plasma 25-hydroxyvitamin D [25-(OH)D] and 1,25-dihydroxyvitamin D [1,25-(OH) 2 D] concentrations were measured in 19 elderly women with, and 21 without, vertebral fractures, before and after treatment with 25-hydroxyvitamin D 3 [25-(OH)D 3 ], to establish whether malabsorption of calcium in elderly women with vertebral fractures has a cause different from that in elderly women without vertebral fractures. 2. Malabsorption of calcium and low plasma 25-(OH)D and 1,25-(OH) 2 D concentrations were common in both groups of women but there was no significant difference in these variables between the two groups. 3. After treatment with 40 μg of 25-(OH)D 3 daily for 7 days, there was a significant increase in plasma 25-(OH)D and 1,25-(OH) 2 D in both groups of women, but radio-calcium absorption increased significantly only in the group without vertebral fractures. 4. Elderly women with vertebral fractures have malabsorption of calcium which is resistant to the action of vitamin D metabolites at concentrations which correct calcium malabsorption in elderly women without vertebral fractures.