An 18-yr-old male developed Mycoplasma pneumoniae pneumonia associated with IgA nephropathy. In the biopsied renal tissue, immunofluorescence showed striking deposits of IgA, C3 and C3 activator in the mesangium, and electron microscopic examination showed electron-dense deposits in the mesangial region. When the patient's serum was preincubated with rabbit antiserum to human IgA, the passive hemagglutination titer for M. pneumoniae was much decreased. We suggest that in some cases, IgA nephropathy may follow infection by an organism which stimulates in IgA antibody response, leading to the formation of IgA antigen antibody immune complex deposits in the glomerular mesangium.