Acquired anti-Factor V deficiency caused by inhibitor production is a rare coagulation disorder. Although this is a well known entity in the literature, choice of optimal treatment for an individual patient is difficult, given that no standard therapeutic measures are available because of rare incidence and various underlying diseases occurring in the elderly. An 88 year-old man treated for Hashimoto's disease was found to exhibit prolongation of both prothrombin time and activated partial thromboplastin time. Detailed study of coagulation factors revealed a deficiency of Factor V. Our patient's coagulation disorder resolved in two weeks with intravenous administration of prednisolone 20 mg/day. Clinical features of autoimmune disease-related Factor V deficiency are discussed, along with eight previously reported cases over the past 20 years.