The determination of beta 2-microglobulin has recently been proposed as a promising diagnostic method to monitor the state of renal allografts. Elevated levels of beta 2MG in the serum and/or urine allow the substantiation of the diagnosis of an acute graft rejection and are helpful in distinguishing acute tubular necrosis from a rejection reaction. In this paper, the usefulness of beta 2MG serum levels is evaluated, not only during the immediate post-operation phase but also for the long-term prognosis of renal allografts. The immunosuppression treatment included methylprednisolone and azathioprine in all the presented patients. The data indicate that a rapid normalization of beta 2MGSL within 6 days, even if the decrease is interrupted by re-elevation due to acute rejection episodes or inflammatory diseases, represents a good long-term prognosis for kidney allografts.