The mechanisms responsible for the formation and development of segmental lesions of IgA nephropathy in children were studied by analysis of three dimensional reconstruction. Forty-eight segmental lesions from 15 cases (diffuse proliferative glomerulonephritis (DPGN) in 13 cases, focal glomerulonephritis (FGN) in 2 cases) were examined by light microscopy by analysis of serial sections (in average 26 sections/glomerulus). In tuft, three types of tuft lesions were defined by their chronisity: 1) endocapillary proliferation including exudative changes, 2) mesangial hypercellularity, 3) deposition of mesangial matrix with sclerosis. Extra-capillary lesions, namely crescent were also defined by their chronisity: 1) cellular, 2) fibrocellular, 3) fibrous. The structural relation in each tuft and endocapillary lesions were observed. Endocapillary proliferative lesions of tufts were closely associated with cellular crescent of extra-capillary lesions. In fibrocellular and fibrous crescents, the frequency of endocapillary proliferation in tufts were reduced, while the association of mesangial proliferation and sclerosis were increased. Nevertheless, endocapillary proliferation of tufts were still observed in 33% of fibrous crescent. We concluded that segmental lesions were originated by endocapillary tuft lesions leading to cellular crescents. The multifocal and repeated attack of endocapillary proliferative lesions within segmental sclerosis promotes further development of glomerular sclerosis in IgA nephropathy in children.