A 72-year-old male presented with oral bleeding resulting from acquired factor V (FV) inhibitor. We observed abnormalities in prothrombin time (PT) (8%) and activated partial thromboplastin time (APTT) (>200 seconds). FV activity was less than 3%, and a mixing test did not correlate with PT. FV inhibitor assay demonstrated 240 Bethesda units/ml. The patient also showed markedly decreased activity of Factors II (13%) and X (14%). Oral bleeding disappeared and coagulation abnormalities improved with prednisolone therapy. High titer FV inhibitor might affect coagulation assays even in a patient with normal factor activity.