Twenty-three obese subjects who had undergone intestinal bypass operation (end-to-side jejunostomy) were studied with respect to vitamin D and other indices of calcium metabolism. Group 1 (11 patients) was examined before and one year after operation. Group 2 (12 patients, bypass operated two to seven years earlier was investigated twice with an interval of one year. The two groups were comparable. Bone mineral content and alkaline phosphatases were unchanged during the study in both groups. Bone phosphorus/hydroxyproline ratio was high postoperatively indicating a high degree of bone mineralisation. Serum calcium declined rapidly in group 1 to a constant level, which was maintained in group 2. The serum levels of iPTH and 1,25(OH)2D did not change within each group, but combining the two groups demonstrated an increase/decrease in iPTH/1,25(OH)2D over the years. The findings suggest that factors other than 1,25(OH)2D and iPTH are involved in calcium metabolism in such patients. The findings do not justify routine administration to such patients of high potency vitamin D derivatives, f.ex. 1 alpha-OH-D3.