Primary IgA nephropathy is the most common form of glomerulonephritis in Australia. The condition presents in a variety of ways, but commonly with synpharyngitic hematuria, most often in young men in the third and fourth decades. The course of the disease is indolent but there is progression to renal failure in up to one quarter of cases. Renal biopsy morphology is variable but the essential immunofluorescence finding is diffuse mesangial IgA staining of greater intensity but often in association with other immunoglobulins. C3 is usually also present. Mesangial cellularity is increased in some two-thirds of cases, one third being of a minor focal or variable extent and one-third diffuse. Focal segmental lesions, hyaline nodules and vascular changes are frequent. Crescents are also often present. The etiology of the disease is uncertain but has been linked with HLA antigens, elevated serum IgA levels, IgA polymers, immune complexes and impaired T cell function. Secondary forms of mesangial IgA deposition occur with mucosal defects, hyperglobulinemia or impaired hepatobiliary clearance, and these may offer some insight into the immunopathogenesis of the primary disease.