Several new findings emerged recently from biochemical, genetic, and molecular studies of patients with IgA nephropathy. It appears that immunoglobulin A1-secreting cells of IgA nephropathy patients produce increased amounts of aberrantly glycosylated IgA1 in which the O-linked glycans in the hinge region are deficient in the content of galactose. The galactose-deficient IgA1 in the circulation is recognized by naturally occurring antibodies with anti-glycan specificity, and immune complexes are formed. These circulating immune complexes escape hepatic degradation and eventually are deposited in the kidney mesangium. Resident mesangial cells bind the IgA-containing immune complexes with the involvement of a novel IgA receptor and become activated. A familial form of IgA nephropathy has been linked to chromosome 6q22-23. Recent progress in molecular analyses of IgA nephropathy thus defines this disease as an autoimmune process with a novel IgA mesangial receptor and certain genetically determined traits.