IgA nephropathy may be associated with colonisation with Haemophilus parainfluenzae. In patients with glomerular diseases, we examined renal-biopsy specimens for presence of bacterial antigen by immunofluorescence microscopy with rabbit antiserum against H parainfluenzae, and by enzyme-linked immunosorbent assay looked for IgA antibody against H parainfluenzae in patient sera. The rabbit antiserum recognised by immunoblotting four components of H parainfluenzae outer membranes (OMHP) of molecular weights 19.5, 30, 33, and 40.5 kDa. All 44 patients with IgA nephropathy and 2 of 39 patients with other glomerular diseases showed mesangial deposition of OMHP antigens (p < 0.001). Patients with IgA nephropathy had significantly more IgA antibody against H parainfluenzae than did patients with other glomerular diseases. IgA antibody in the sera of patients with IgA nephropathy recognised by immunoblotting the same four components of OMHP as recognised by rabbit antiserum. Glomerular deposition of OMHP antigens and the presence of IgA antibody against OMHP in patients with IgA nephropathy suggest that H parainfluenzae has a role in the aetiology of this disease.