BACKGROUND: Acute rejection (aRx) has a major impact on the long-term outcome of renal allografts, and its diagnosis is contingent on the invasive procedure of allograft biopsy. New immunosuppressive protocols have reduced the incidence but have not abolished this problem. Moreover, aRx is now more frequently seen several weeks after transplantation in outpatients. A noninvasive diagnostic test for predicting aRx could improve the management and outcome. The recently described measurement of urinary mRNA expression offers a new noninvasive approach. METHODS: In this study, the authors monitored the urinary mRNA expression (221 specimens from 26 patients) of various immune molecules by real-time reverse-transcriptase polymerase chain reaction for up to 3 months after kidney transplantation. Most of the patients received anti-interleukin (IL)-2 receptor monoclonal antibody induction and tacrolimus-based maintenance immunosuppression, which resulted in a low incidence of aRx. To verify the "rejection" markers, an additional nine samples of patients with aRx were analyzed. RESULTS: Granulysin mRNA increase (vs. 95% confidence interval of 159 urine samples from nonrejecting patients) was detected during 11 of 14 aRx episodes, and follow-up studies showed its predictive value for delayed aRx episodes, even weeks before enhanced serum creatinine was observed. Granulysin induction was associated with enhanced regulated on activation normal T-cell expressed and secreted (RANTES) mRNA expression in 8 of 11 samples. Other cytotoxic effector molecules (granzyme B, perforin, FasL), cytokines (tumor necrosis factor-alpha, RANTES, IL-2, IL-10, interferon-gamma, transforming growth factor-beta), CD3, and CCR1 showed less specificity and sensitivity. CONCLUSIONS: The authors' data illustrate that the noninvasive kinetic mRNA expression measurement of defined markers in urinary cells of renal allograft recipients allows the early noninvasive detection of ongoing aRx.