The detection of anti-neutrophil cytoplasmic antibodies (ANCA) is now a routine part of the evaluation of patients clinically suspected of suffering from small vessel vasculitis. The factor(s) that trigger the development of these autoantibodies and their role in the pathogenesis of vasculitis is still unclear. We describe four patients who presented to us since June 1990. All patients had positive ANCA serology and had clinical evidence of vasculitis. In all patients soon after the establishment of ANCA positivity, a carcinoma of either the respiratory or urinary tracts was diagnosed. We suggest that in some cases of ANCA-associated vasculitis, malignant disease may be a trigger for either the generation of these autoantibodies, or the development of vasculitis.