Microvasculitis may play a greater part in the pathogenesis of paraproteinaemic neuropathies than is generally recognised, producing tissue destruction by convergent immune and physical mechanisms. We present a patient with a clinical syndrome of mononeuritis multiplex and a circulating IgM lambda paraprotein, in whom bone marrow aspiration revealed a lymphoplasmacytoid lymphoma. Microvasculitic changes were present in the first nerve biopsy, and the second showed extensive destruction of neural architecture and deposition of IgM-related material. A 2-stage pathogenic cascade is postulated and explored with a review of the relevant literature.