A 53-year-old man, who had been diagnosed with mild hemophilia A (HA) at 35 years of age, was hospitalized with a thigh hematoma. His bleeding continued despite the administration of recombinant factor VIII (FVIII). The results of an FVIII/von Willebrand factor binding assay were normal. The patient's FVIII coagulant activity (FVIII:C) was low, but his FVIII antigen levels were within the normal limits, suggesting FVIII protein dysfunction. The FVIII:C measurements obtained by one-stage clotting and chromogenic assays were different. An FVIII gene analysis revealed a missense mutation p.Ser308Leu, which is rare in Japan. This case highlights that gene analyses and chromogenic assays are necessary to interpret the discrepancies between FVIII:C and the bleeding phenotype of patients with mild HA.