BACKGROUND: Recently we reported a possible role for secretory IgA (SIgA) in IgA nephropathy (IgAN), as suggested by increased serum levels in patients with active disease and accumulation of SIgA in a glomerular eluate. Therefore, we attempted to find support for these findings by analysis of the presence of SIgA in biopsies of IgAN patients. METHODS: Renal biopsies of 26 patients with biopsy-proven IgAN were analysed for the presence of SIgA and complement proteins. RESULTS: In 15% mesangial deposition of SIgA was demonstrated, using a specific staining for secretory component (SC) and colocalization with IgA. The presence of SIgA in these biopsies showed a strong correlation with deposition of mannose-binding lectin (MBL) and C4d. Moreover, we observed a strong colocalization between SIgA and MBL or C4d. This local complement activation has previously been linked to more severe renal disease. CONCLUSIONS: Therefore, these data provide additional evidence for a pathogenic role for SIgA in IgA nephropathy.