Tonsillar abnormalities have been observed in patients with IgA nephropathy (IgAN). In addition, it has been suggested that the reticulization of tonsillar crypt epithelium is important in tonsillar immunity. Therefore, we investigated reticulization of this area in patients with IgAN (10 cases) and compared results with those of control patients who exhibited recurrent tonsillitis or tonsillar hypertrophy (12 cases). Immunohistochemical staining of anti-keratin antibody PKK1 was employed to visualize reticulization. Tonsils of controls showed well developed reticular crypt epithelia with lymphoepithelial symbiosis, and the non-reticulated area was less than 7% (mean 1.9%, range 0-6.3%) of the total crypt epithelia per overall section. In IgAN tonsils, however, non-reticulated crypt epithelium was frequently observed and, in the advanced stage of IgAN, exceeded 50% (mean 62%, range 50-69%) of total crypt epithelia. The extent of glomerular damage in IgAN patients correlated significantly with the percentage of the area of non-reticulated crypt epithelia in palatine tonsils with a correlation coefficient of 0.91. Considering together with our finding that PKK1-positive epithelial reticular cells strongly expressed HLA-DR antigens in normal tonsils, the low level of reticulization in IgAN patients may induce the unusual immunity responsible for the pathogenesis of IgAN.