In order to investigate the participation of monocytes/macrophages(Mo/M phi) in the progression of various kidney diseases of children, Mo/M phi in urine and that infiltrating renal tissue were both measured as the number of CD68 positive Mo/M phi (CD68+ Mo/M phi), using anti-CD68 antibody. The number of CD68+ Mo/M phi infiltrating in one glomerulus was significantly higher in Henoch-Schönlein purpura nephritis(HSPN) (p < 0.01) in comparison with that in minimal change nephrotic syndrome(MCNS) (p < 0.01), and a high tendency was found in IgA nephropathy (IgAN), proliferative glomerulonephritis (non-IgAN), focal segmental glomerulosclerosis(FSGS) and membranoproliferative glomerulonephritis (MPGN), respectively. The number of CD68+ Mo/M phi infiltrating one mm2 of tubulo-interstitium area was significantly higher in HSPN(p < 0.05), FSGS(p < 0.01), Alport's syndrome(p < 0.01), respectively, than that in MCNS. The number of CD68+ Mo/M phi in one milliliter of urine correlated significantly with both that infiltrating the glomerulus and the tubulo-interstitium(both p < 0.01). Moreover the number of urine CD68+ Mo/M phi in a clinically active stage was significantly higher than that in an inactive stage in the AGN(p < 0.05), IgAN(p < 0.05), HSPN(p < 0.05), non-IgAN(p < 0.01) and MPGN groups(p < 0.05), respectively. From these results, 1) It was suggested that the Mo/M phi infiltrating renal tissue participated in the development of various kidney diseases. 2) It was predicted that CD68+ Mo/M phi in urine reflected both the number of Mo/M phi infiltrating the glomerulus and that in the tubulo-interstitium. 3) It was suggested that the number of CD68+ Mo/M phi in urine indicated clinical activity in proliferative glomerulonephritis groups of children.