The pathological findings of 13 patients with immunoglobulin A (IgA) nephropathy or Henoch-Schönlein purpura nephritis before and after urokinase (UK) administration were investigated retrospectively. The mean activity index value decreased significantly at the time of the second biopsy compared with that of the biopsy before UK treatment. The mean chronicity index value, which was considered to reflect the renal outcome, before and after UK treatment did not change significantly, although it improved significantly in six patients. Immunofluorescence microscopy showed that the immune deposition of C3 decreased, but that of IgA and fibrin-related antigen were unchanged, by UK therapy. These results suggest that UK may prevent the mesangial proliferation associated with IgA nephropathy and Henoch-Schönlein purpura nephritis not only by its fibrinolytic action, but also by other mechanisms, such as digestion of the mesangial matrices.