Background. Sera of IgA nephropathy (IgAN) patients contain high levels of circulating immune complexes composed of IgA1 molecules with aberrantly glycosylated hinge-region O-linked oligosaccharides and IgG or IgA1 antibodies with anti-glycan or anti-hinge-region peptide specificities. Due to damaged sieving properties of the glomerular capillary wall in IgAN, these immune complexes may appear in the urine. Methods. We collected urine samples from 29 patients with biopsy-proven IgAN (Group I), 27 proteinuric patients with non-IgA nephropathies (Group II) and 28 healthy volunteers (Group III). The levels of urinary IgA and IgG and IgA-IgG-containing immune complexes were measured by ELISA and standardized for urinary creatinine concentrations. Results. The urinary IgA and IgG levels were significantly higher in Groups I and II than in Group III. Although the excretion of IgA as a fraction of total urinary protein was not significantly greater in IgAN patients than in patients with other renal diseases, the excretion of aberrantly glycosylated IgA1 was observed by western blot in 68% of the IgAN patients but in none of the healthy controls. The urinary levels of IgA-IgG immune complexes were significantly higher in Group I than in Groups II (P < 0.01) and III (P < 0.05). There was no significant difference in the levels between Groups II and III. These immune complexes had a molecular mass between 650-850 kDa, as shown by size-exclusion chromatography. Conclusion. The amounts of urinary IgA-IgG-containing immune complexes were significantly higher in patients with IgAN than in patients with non-IgA nephropathies or healthy controls. © 2006 Oxford University Press.