A review is presented of the current knowledge concerning the syndrome of IgA nephropathy. Primary and secondary forms can now be delineated and this division has improved understanding of immunopathogenetic mechanisms giving rise to glomerular mesangial IgA deposits. Attention is paid to disorders of antigen exclusion at mucosal surfaces, defective reticulo-endothelial sequestration, and altered immunoglobulin A production and regulation. Particular reference is made to these mechanisms with respect to primary IgA nephropathy, Henoch-Schoenlein purpura and mesangial IgA nephritis associated with alcoholic cirrhosis.