['CD103 is displayed on the cell surface of alloreactive CD8 cytotoxic T lymphocytes (CTLs) and is a critical component for the intraepithelial homing of T cells. Because intratubular localization of mononuclear cells is a feature of acute cellular rejection of renal allografts, we explored the hypothesis that CD103 messenger (m)RNA levels in urinary cells predict acute rejection.', 'We collected 89 urine specimens from 79 recipients of renal allografts. RNA was isolated from the urinary cells, and we measured CD103 mRNA levels and a constitutively expressed 18S ribosomal (r)RNA with the use of real-time quantitative polymerase chain reaction assay.', 'CD103 mRNA levels, but not 18S rRNA levels, were higher in urinary cells from 30 patients with an episode of acute rejection (32 biopsies and 32 urine samples) compared with the levels in 12 patients with other findings on allograft biopsy (12 biopsies and 12 urine samples), 12 patients with biopsy evidence of chronic allograft nephropathy (12 biopsies and 12 urine samples), and 25 patients with stable graft function after renal transplantation (0 biopsies and 33 urine samples) (P = 0.001; one-way analysis of variance). Acute rejection was predicted with a sensitivity of 59% and a specificity of 75% using natural log-transformed value 8.16 CD103 copies per microgram as the cutoff value (P = 0.001).'] Do cD103 mRNA levels in urinary cells predict acute rejection of renal allografts?