Mononeuritis multiplex is known to occur in many illnesses including certain types of systemic vasculitis. Anti-neutrophil cytoplasmic autoantibody (ANCA) has been described in association with Wegener's granulomatosis, polyarteritis nodosa, pauci-immune necrotizing crescentic glomerulonephritis, and so-called ANCA-associated vasculitis. We describe three patients who presented with mononeuritis multiplex and positive tests for ANCA. A careful search revealed underlying vasculitis in two of the three patients. Whereas both respiratory and renal involvement are well known in ANCA-associated vasculitis, to our knowledge the relationship of mononeuritis multiplex and ANCA positivity has not previously been described. The three patients were treated with steroids and oral cyclophosphamide. All demonstrated marked clinical improvement, as well as a decrease or disappearance of the ANCA. Since mononeuritis multiplex may be a presenting symptom of many illnesses, a serological marker may be helpful for early diagnosis and prompt treatment.