Three patients with myasthenia gravis, in whom a thymoma had been removed during total thymectomy four to 14 years earlier, developed glomerulonephritis presenting as the nephrotic syndrome. All had been treated previously by plasma exchange and were receiving azathioprine. Two were also taking prednisolone. Serum immune complexes containing IgM were present in all three. Renal histology showed either focal segmental glomerulosclerosis, minimal change lesion or focal proliferative glomerulonephritis. No immunoglobulin deposits were seen. Two patients lost their proteinuria on high dose corticosteroid therapy and their renal function improved. Glomerulonephritis occurred in nine per cent of patients with thymoma receiving immunosuppressive treatment with azathioprine for more than two years. We suggest that this treatment may be implicated in its aetiology.