Osteoporosis is a common, and potentially severe, complication seen in the heart transplant recipient. Frequently there is loss of bone mineral density prior to transplant that begins the downward spiral to fractures of the femoral neck and vertebrae. Multiple factors are responsible for the development of osteoporosis posttransplant. These factors include pretransplant bone mineral loss, use of corticosteroids and cyclosporine posttransplant, and development of persistent hypomagnesemia posttransplant. This article explores the relationship of serum magnesium levels to maintenance of bone mineral density.