IgA nephropathy is generally considered to be an immune-complex-mediated or aggregated (polymerized) IgA (IgA1)-mediated glomerulonephritis. Since the pathogenesis of IgA nephropathy is still obscure, it is important to determine the initiation and progression of this disease using the spontaneous animal model. The ddY mouse strain can serve as a spontaneous animal model for IgA nephropathy. Genetic factors are considered to be involved in the initiation and progression of IgA nephropathy. It has been hypothesized that susceptibility genes for IgA nephropathy can be detected by a genome-wide scan using this model. The peak marker D10MIT 86 on chromosome 10 is located on the region syntenic to human 6q22-23 with IGAN1, which is responsible for familiar IgA nephropathy. There are several developmental and/or exacerbating factors in this disease. Among them, the loss of glomerular epithelial cells (podocytes) and interstitial mast cell infiltration are important factors for progression of glomerulosclerosis and tubulointerstitial injury in patients with IgA nephropathy.