Although liposome-encapsulated clodronate has been used as a means to deplete macrophages from certain tissues, target leukocyte subtypes within the kidney are not clearly known under normal and pathologic conditions. The present study was therefore conducted to examine the effects of liposome clodronate on renal infiltrating cell type following unilateral ureteral obstruction (UUO) and tried to correlate these changes to the mechanisms of early development of renal fibrosis. Renal infiltrating leukocyte subtypes and counts were determined by using multicolor flow cytometric analysis of cell suspensions from obstructed kidneys. UUO for 5 days elicited renal tubular apoptosis and renal fibrosis and showed 4-fold increase in renal leukocytes including monocytes/macrophages, dendritic cells, and T-cells. Repeated administration of liposome clodronate selectively depleted F4/80+ monocytes/macrophages and F4/80+ dendritic cells but not F4/80(-) dendritic cells or other cell types in both obstructed and non-obstructed kidneys. Tubular apoptosis and renal fibrosis were also significantly attenuated by liposome clodronate. Increased gene expression of TNF-alpha and TGF-beta observed in obstructed kidneys were markedly attenuated by depletion of renal mononuclear phagocytes. These findings suggest that F4/80+ monocytes/macrophages and/or F4/80+ dendritic cells play a pivotal role in the development of obstruction-induced tubular apoptosis and renal fibrosis, possibly through TNF-alpha and TGF-beta dependent mechanisms.