We describe an otherwise healthy 3-year-old girl who presented with ecchymoses and epistaxis after diarrhea. Her past medical history and family history were negative for bleeding disorders. Complete blood cell counts, including platelet count, were normal. Prothrombin time (PT, INR 2.64) and activated partial thromboplastin time (aPTT, 68.7sec.) were markedly prolonged. A mixing test was performed and aPTT did not normalized with a 1:1 dilution of patient and normal human plasma. Hypocomplementemia and lupus anticoagulant (LA) were detected but other serologic tests for autoimmune diseases were negative, including anti-nuclear antibodies, anti-DNA antibody and anti-cardiolipin antibody (aCL) . Further coagulation studies demonstrated the presence of hypoprothrombinemia and phosphatidylserine-dependent anti-prothrombin antibody (aPS/PT). Adenovirus infection was diagnosed serologically. Her clinical symptoms resolved spontaneously without specific treatment. All coagulation abnormalities normalized and aPS/PT disappeared within six months. Nine months later, the child is asymptomatic, with no clinical or laboratory evidence of an underlying disease. The bleeding tendency in this case was associated with acquired hypoprothrombinemia caused by transient anti-prothrombin antibody following adenovirus infection., 東京女子医科大学東医療センター小児科開局40周年記念論文集