Background. Urinary mononuclear cells (UMCs) were studied in IgA nephropathy (IgAN) in relation to clinical and histopathological parameters. Methods. A two-color flow-cytometry analysis was used to phenotype and quantitate UMCs in patients with IgAN (n = 37) and those with other kidney diseases (n = 27). Results. The IgAN patients had higher numbers of UMCs than the patients with other kidney diseases. Macrophages (CD14+
P &lt
0.0001) and T lymphocytes (CD3+
P = 0.0001
CD4+, P &lt
0.0001) were predominantly found. In the IgAN patients, the quantities of urinary CD3+, CD14+ cells correlated with impaired renal function
namely, 'serum creatinine, creatinine clearance, and urinary protein excretion. The UMCs were associated with histopathological changes such as glomerular segmental lesions (r = 0.511
P &lt
0.01 for CD3+
r = 0.623, P &lt
0.0001 for CD4+, and r = 0.552
P &lt
0.001 for CD14+) and the index of glomerular hypercellularity (r = 0.405
P = 0.01 for CD3+ T cells and r = 0.392, P = 0.01 for CD14+ macrophages). Moreover, the UMCs reflected the severity of pathology, as estimated by the glomerular score (r = 0.424
P &lt
0.01 for CD3+
r = 0.458
P &lt
0.01 for CD4+
r = 0.500, P = 0.001 for CD14+ cells). UMCs were weakly correlated with tubulointerstitial changes. Conclusion. UMCs were associated with the clinical stage of IgAN and mirrored the extent of histopathology in the kidney. Flow-cytometry analysis of UMCs may help to predict the course of the disease.