Although more than 20 years have passed since the initial report of IgA nephropathy, the etiology of this disease is still unclear. Some reports suggest that the tonsil is an important etiological factor. We performed tonsillectomy on 26 patients with IgA nephropathy associated with chronic tonsillitis, and followed up the patients for two years after the operation to evaluate its clinical effect on this disease. Twelve patients (efficacy rate 46%) showed distinct improvement in urinary findings after the operation, although the efficacy rate went down as renal injury advanced. Serum IgA levels decreased significantly after the operation both in patients who improved and in those who did not; the decrease was especially evident in patients who had high levels of serum IgA before tonsillectomy. In 4 patients who improved, the level of circulating immune complex (CIC) was extremely high before, and decreased significantly after, the operation. One patient suffered renal failure three years after tonsillectomy. When renal injury has advanced to the clinically apparent degree, as occurred in this patient, tonsillectomy is absolutely contraindicated. In reaching a decision as to whether tonsillectomy is indicated in mild cases, the change in the number of erythrocytes in urinary sediments may be a sensitive parameter of the tonsillar provocation test.