Calcium homeostasis during a very-low-calorie diet (VLCD) was examined. During the treatment, intact parathyroid hormone tended to decrease initially, bone Gla-protein increased significantly in the third week, and tartrate-resistant acid phosphatase decreased during the entire treatment. Serum ketone bodies showed significant correlations with intact parathyroid hormone and bone Gla-protein in some cases. Regarding bone mineral content, bone mineral content of the head increased while that of the legs decreased, resulting in no significant changes in total bone mineral content. These results suggest that VLCD treatment alters calcium homeostasis, which may cause regional bone mineral changes.