Decreases in transplant function may be attributable to a variety of conditions, including prerenal and postrenal failure, cyclosporin A (CsA) toxicity, polyoma nephritis, recurrent glomerulonephritis, and rejection. The diagnosis of rejection should therefore be made on the basis of a transplant biopsy of adequate size, before the initiation of any therapy. Pulse steroid treatment (three to five 0.25- to 1.0-g pulses of methylprednisolone, administered intravenously) is the usual first-line therapy and has a 60 to 70% success rate, although orally administered prednisone (0.25 g) may be just as efficacious. Even if reverted, any rejection should trigger an at least temporary increase in basal immunosuppression, consisting of an increase in CsA or tacrolimus target levels, the addition of steroids or an increase in their dosage, the addition of mycophenolate mofetil, or a switch from CsA to tacrolimus. The addition of rapamycin or its RAD derivative may fulfill the same purpose. Steroid resistance should not be assumed before the fifth day of pulse steroid treatment, although histologic features of vascular rejection may indicate the need for more aggressive treatment earlier. Steroid-resistant rejection is traditionally treated with poly- or monoclonal antilymphocytic antibodies, with success rates of 60 to 70%. Their potential benefit must be carefully balanced against the risks of infection and lymphoma. More recently, mycophenolate mofetil has been successfully used to treat steroid-resistant rejection, but only of the interstitial (cellular) type. Switching from CsA to tacrolimus for treating recurrent or antibody-resistant rejection is successful in approximately 60% of cases. Plasmapheresis and intravenously administered Ig have been used in some desperate cases, with surprising success. Because none of the available drugs has a significantly better profile of therapeutic versus adverse effects, the possible benefits of continued rejection therapy must be continuously balanced with the potential for serious, sometimes fatal, side effects.