Leukocyturia is associated with postinfectious glomerulonephritis (GN), interstitial nephritis, and renal allograft rejection. In addition, prominent infiltration of T cells and macrophages is commonly observed in the renal tissues of patients with GN, accompanied by cellular crescent formation and/or interstitial cell infiltration. Because these infiltrating T cells were thought to participate in the development of the diseases and to appear in the urinary space while functioning as effector cells in the renal inflammatory lesion, the study focused on the characterization of T cells that appeared in urine. Freshly voided urine cells were analyzed by flow cytometry to determine their phenotype and by reverse transcriptase-PCR to detect cytokine mRNA. In urine from patients with different forms of GN, including IgA nephropathy, Henoch-Schönlein purpura nephritis, and anti-neutrophil cytoplasmic antibody-associated GN, T cells appeared together with macrophages. The urine T cells were mainly CD45RA(-), CD45RO(+), and CD62L (L-selectin)(-), which are the phenotypic features of effector T cells. In agreement with this finding, T cells infiltrating glomeruli, crescents, and tubulointerstitial lesions were also effector type. Moreover, these urine cells expressed mRNA of the T helper lymphocyte 1 cytokines, interleukin-2, and/or interferon-gamma. These findings suggest that the appearance of effector T cells in urine may reflect the cellular immune reaction that occurs in the kidneys of patients with GN accompanied by active cell infiltration.