Numerous studies in the literature report aberrant immune responsiveness in immunoglobulin A (IgA) nephropathy. However, all these studies investigate immune responses invoked by an artificially engineered antigen challenge. For the first time in IgA nephropathy, we report the systemic humoral responses generated as part of an active mucosal immune response against a common environmental pathogen, Helicobacter pylori (Hp). We studied 22 patients with IgA nephropathy and 9 controls without renal disease who were shown to be infected with Hp, using a 13C-urea breath test. Hp antigen-specific enzyme-linked immunosorbent assays were established to measure the anti-Hp IgA, IgG, and IgA and IgG subclass antibody levels. In addition, anti-Hp responses in the monomeric and polymeric (pIgA) fractions of serum IgA were measured after separation by gel filtration high-performance liquid chromatography. IgA nephropathy was associated with both a greater rate of IgA anti-Hp seropositivity (P < 0.05) and a more pronounced IgA anti-Hp antibody response (P < 0.01). In almost all cases, IgA anti-Hp was IgA1, and more than 90% was polymeric. There was no difference in the frequency of IgG anti-Hp seropositivity, but patients produced a much greater IgG anti-Hp response (P < 0.01). In addition, the IgG subclass responses were markedly different, with IgG1 predominant in controls and IgG2 and IgG3 the major subclasses produced in IgA nephropathy. We have shown an exaggerated systemic antibody response to mucosal infection caused by Hp in patients with IgA nephropathy, predominantly consisting of pIgA1, IgG2, and IgG3. This suggests that in IgA nephropathy, not only is pIgA1 production poorly controlled, but regulation of IgG isotype switching in response to mucosal pathogens is also deranged.