We will enroll patients who have undergone cardiac transplantation into a randomized, double-blind, 12-month study of the efficacy and safety of calcitriol (Rocaltrol) and alendronate sodium (Fosamax) in the prevention of bone loss after transplantation. We will give all participants standard pre- and post-transplantation management and immunosuppressive therapy, three tablets of calcium citrate (Citracal + D, each containing 315 mg of elemental calcium and 200 IU of vitamin D), and a multivitamin providing 400 units of vitamin D daily. We will randomize participants to one of two active treatment groups within 1 month of transplantation. We will give Group A active alendronate (10 mg/day) and placebo calcitriol. We will give Group B placebo alendronate and active calcitriol (0.25 micrograms BID). The primary efficacy endpoint is the change in spine bone mineral density (BMD) during the first 6 months after transplantation. The secondary efficacy endpoint is the change in hip BMD during the first year after transplantation. We will also monitor the incidence of vertebral fracture.