Fifty consecutive cases of nerve and muscle microvasculitis (MV) seen on nerve and muscle biopsies were studied. These were observed in a 5 years period, among 1076 nerve and/or muscle biopsies performed in adult patients in the Laboratoire de Neuropathologie Charles Foix. The systemic necrotizing vasculitides, in which the arteries of diameter greater than 70 microns are involved, acute polymyositis, sarcoidosis and acute polyneuritis were not considered in this study. Mononuclear cell infiltration was the rule. It was associated to leukocytoclasis in 2 cases. No fibrinoid necrosis was seen. These changes were highly diagnostic when seen in the nerve or the connective tissue of the epi or perimysium. The etiology of these microvasculitides was mainly connective tissue diseases (42 p. 100) and, overall, panarteritis nodosa (16 p. 100), or malignancies (28 p. 100) which comprise 7 solid tumors and 4 lymphomas. Other cases were related either to systemic diseases (thromboangiitis obliterans, monoclonal dysglobulinemia, cholesterol embolus) or to local trauma. The relationship between MV and peripheral neuropathy was less obvious in 3 cases of mononeuritis multiplex associated with diabetes mellitus and in 3 cases of acute idiopathic and regressive mononeuritis multiplex. In 5 cases, no cause was found.