Organ transplantation offers new life to patients who suffer from incurable disease. The problem of rejection of the transplanted organ has been overcome with the use of potent immunosuppressive drugs. These drugs, although they allow graft tolerance and graft survival, also are associated with complications such as osteoporosis. Although factors such as nutrition, gonadal status, and ambulatory status are important, the use of immunosuppressive drugs appears to be the main factor in the development of osteoporosis. The drugs that are responsible for this bone loss are glucocorticoids and the calcineurin phosphatase inhibitors, cyclosporine and tacrolimus. The incidence of bone disease depends, in part, on which organ is transplanted. Kidney transplant recipients appear to be less susceptible to the development of overt osteoporosis than do heart or liver transplant recipients. The most critical period of bone loss in organ recipients appears to be within the first 6 months, with the most dramatic reduction occurring within the first 3 months following transplantation. Trabecular (cancellous) bone of the spine appears to be most at risk, with vertebral fractures occurring most commonly. Transplant recipients should be evaluated by bone mineral densitometry and measurement of vitamin D metabolites, blood urea nitrogen, creatinine, calcium, and phosphate. Markers of bone turnover may help in assessing the rate of remodeling. Gonadal function should be ascertained by measurement of serum testosterone (males) or estradiol (females) levels. Therapy should be directed toward prevention of bone loss as well as helping to restore what already may have been lost. Administration of calcium and vitamin D and sex hormone replacement, if indicated, should be considered.(ABSTRACT TRUNCATED AT 250 WORDS)