Background: Sera of IgA nephropathy patients display increased levels of circulating immune complexes which contain IgA. Due to damaged sieving properties of the glomerular capillary wall these immune complexes may appear in the urine. Aim: The objective of this study was to find out an amount of IgA-containing complexes in the urine of IgA nephropathy patients. Methods: We collected urine samples from 29 patients with biopsy-proven IgA nephropathy (Group I), 27 proteinuric patients with non-IgA nephropathies (Group II), and 28 healthy volunteers (Group III). The amounts of urinary IgA-containing immune complexes were investigated by ELISA method. Results: The amount of urinary IgA-containing immune complexes was significantly higher in Group I than in Groups II and III, while no difference in their levels was found between Groups II and III. Conclusion: Significantly larger amount of IgA-containing immune complexes is present in the urine of IgAN patients than in the urine of patients with other proteinuric renal diseases or than in healthy individuals. The increased levels of urinary IgA-containing immune complexes contribute to the kaleidoscope of aberrant IgA metabolism in IgA naphropathy patients. Their increased levels support diagnosis of IgA nephropathy before renal biopsy is performed.