Twins with parallel loss of kidney function and moderate hyperparathyroid bone disease were participants in a double-blind study where twin A was given placebo and twin B calcitriol. After 8 months. A's bone disease had not improved, while B's bone had normalized. Thereafter, both received calcitriol until kidney transplantation 11 months later, when both had normal bone structure. Two years after transplantation, both twins had hyperparathyroid bone disease, but A had more pronounced changes. This report illustrates our findings in larger series: When started early in the course of renal failure, calcitriol can reverse pre-transplant hyperparathyroid bone disease and also influence post-transplant bone disease.