Serum creatinine level is used as a major measure of posttransplant renal function at most centers. A significant elevation of creatinine level suggests allograft rejection. However, other factors affect renal function in the transplant recipient and each may cause an elevation in serum creatinine level, suggesting a rejection episode. It is important to make the correct diagnosis and not treat these episodes with antirejection therapy. The authors reviewed the course of patients transplanted between 1969 and 1974 to determine the pathogenesis of creatinine elevations retrospectivelyfound to be due to causes other than rejection. Six distinct causes were found: hyperglycemia, ureteral obstruction, infection, lymphocele, arterial stenosis, and recurrence of the original disease. Each of these is discussed individually. In order to make the diagnosis of pseudorejection, a high index of suspicion is necessary.