A mild self-limiting mesangial proliferative glomerulonephritis is described in 3 patients with selective IgA deficiency (less than 0.05 g/l). In all cases there was irregular thickening of peripheral glomerular capillary loops and paramesangial deposits. Arteriolar hyalinosis was present in two of the three cases. By direct immunofluorescence, and immunoperoxidase techniques in one case, IgM was present in the glomeruli and C3 was present in blood vessel walls. Secretory component was not detected in the glomeruli of any of the patients. The polyethylene glycol (PEG) precipitation immune complex assay was positive for IgM and IgG immune complexes in one patient and weakly positive for IgM immune complexes alone in another. The serum of one patient contained milk precipitins. All three patients had suffered from recurrent upper respiratory tract infections. It is suggested that the glomerular IgM immune complex deposition is related to the IgA deficiency and that, although the immune complexes may be a consequence of antibodies to dietary bovine proteins, this could be due to recurrent upper respiratory tract infections. This type of glomerulonephritis appears to resemble IgM associated glomerulonephritis in some aspects. Careful appraisal is required to establish if this association constitute a specific entity.