Since cyclosporine (CsA) was introduced into clinical practice in late 1983 to prevent rejection in transplant patients, there has been an almost explosive growth in the number and types of transplants and the number of transplant centers, an increase in the life expectancy of the transplanted organ, and substantial decreases in rates of acute rejection and life-threatening infections. Despite these successes, major improvements in immunosuppressive therapy are needed, especially a reduction in toxic side effects and a rigorous definition of the relation between drug concentration and clinical effects. Such improvements may be achievable with the incorporation of new drugs such as tacrolimus and mycophenolate mofetil into immunosuppression protocols and the development of rigorously defined therapeutic drug-monitoring programs.