Case 1: A 55-year-old woman with hematuria showed marked prolongation of both APTT and PT. Clotting time was corrected in a cross mixing test (CMT). Closer examination revealed that her factor V activity (FV:C) was 3.0% and thus, she was diagnosed with congenital FV deficiency. Afterward, she suffered from right thigh hematoma. Clotting time failed to be corrected in another CMT, and 2.0 BU/mL of FV inhibitor was detected. Finally she was diagnosed with acquired FV inhibitor. Case 2: An 84-year-old woman was admitted to a neighboring hospital because of a urinary tract infection. Screening tests showed prolonged APTT and PT. Clotting time was corrected in a CMT and FV:C was 15.9%; she was thus diagnosed with congenital FV deficiency. After her discharge, she developed intra-articular bleeding. Another CMT revealed that clotting time was not corrected, and 5.0 BU/mL of FV inhibitor was detected. She was diagnosed with acquired FV inhibitor. These patients were initially diagnosed with congenital deficiency due to the deficiency pattern of CMT. Therefore additional CMT and inhibitor assays may be important to for accurate diagnosis whenever a lower FV:C is found. In addition, steroid was effective for the disappearance of inhibitor in both cases.