Mesangial IgA depostiiton was detected by routine fluorescent microscopy of 19 cases of glomerulonephritis. Twelve patients had a variable degree of diffuse mesangial proliferative glomerulonephritis. The heterogeneous histological findings in the other seven patients show that mesangial IgA deposition alone does not have diagnostic specificity. However, when interpretation of the mesangial IgA deposition is complemented by clinical and histological information, a group of patients with the characteristic combined features can be distinguished.