We describe a patient with solitary plasmacytoma of the skull, in whom mononeuritis multiplex was the presenting manifestation. Some features of the POEMS syndrome (plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, monoclonal [M] protein, skin changes), including thrombocytosis, were found. Muscle and nerve biopsies disclosed a small vessel hypersensitivity-type vasculitis and complement-fixing immune complex deposits in vessel walls. Removal of the plasmacytoma resulted in clinical improvement and clearance of the vasculitis and immune complex deposits.