To elucidate the role of monocyte chemotactic and activating factor (MCAF)/monocytechemoattractant protein (MCP) -1 in the pathogenesis of rapidly progressive glomerulonephritis (RPGN), we determined the urinary levels of MCAF/MCP-1 in 20 healthy subjects, 30 patients showing RPGN with crescents, and 39 patients with various types of renal diseases without crescents. We divided RPGN into two subgroups, the acute type and the insidious type, with regard to the declination rate of reciprocals of serum creatinine with time as previously reported. In addition, we divided the patients with RPGN into anti-neutrophil cytoplasmic antibody (ANCA)-related diseases and immune complex (IC)-mediated diseases with regard to etiology. Urinary levels of MCAF/MCP-1 were significantly higher in patientswith RPGN as compared with those of other renal diseases and healthy volunteers (21.8±4.5 vs. 11.6±3.5, 1.0±0.1 pg/ml ⋅ creatinine, respectively, p<0.01, mean±SEM). There was no difference in the urinary levels of MCAF/MCP-1 between the acute and insidious types of RPGN patients. In addition, there was no difference in the urinary levels of MCAF/MCP-1 between the patients with ANCA-related and IC-mediated diseases. Urinary levels of MCAF/MCP-1 in patients with RPGN were correlated well with the percentage of both total crescents and fibrocellular/fibrous crescents and the number of CD68 positive infiltrating cells in the interstitium. Immunohistochemical examinations revealed that MCAF/MCP-1-positive cells were detected in tubular epithelial and endothelial cells and mononuclear infiltrated cells in the interstitium. Moreover, elevated urinary MCAF/MCP-1 levels in patients with RPGN, regardless of subgroups, were dramatically dereased during methylprednisolone pulse therapy induced convalescence. These results suggest that MCAF/MCP-1 may be involved in the pathogenesis of RPGN via macrophage recruitment and activation.