We report an unusual case of a 74-year-old male who developed a serum autoantibody reactive with human coagulation factor V (FV) in an activated form, as demonstrated by coagulation studies and immunoblotting analysis. Despite marked prolongation of a prothrombin time and an activated partial thromboplastin time in this patient, the inhibitor was not associated with clinical bleeding but with multiple cerebral infarctions. The patient had suffered from Sjögren's syndrome with polyclonal hypergammaglobulinaemia. The patient's purified IgG, an immediately acting inhibitor to FV, reacted with a light chain of thrombin-activated FV (FVa) and inhibited the procoagulant activity of FVa without affecting the cleavage of FVa by activated protein C. The FV inhibitor may arise from activation of FV with consequent exposure of neoantigen during the activation of coagulation cascade in the patient with an autoimmune disorder for the background.