Acute rejection (AR) and urinary tract infection (UTI) continue to plague renal transplantation. We tested the hypotheses that UTI does not increase granzyme B mRNA levels in urinary cells, and that the levels distinguish AR from UTI. We measured the levels of granzyme B mRNA in 15 urine specimens from renal allograft recipients with UTI, 29 specimens from patients with AR but without UTI, and 14 specimens from patients without AR and without UTI. We also measured transcript levels in urine specimens from 41 nontransplant individuals, 11 with UTI and 30 without UTI. UTI did not increase granzyme B mRNA levels. Granzyme B mRNA levels were lower in renal allograft recipients with UTI compared with those with AR (P<0.0001). We conclude that bacterial UTI is unlikely to confound AR diagnosis made by measurement of granzyme B mRNA levels in urinary cells.