The many causes of hemarthrosis include acquired hemophilia due to production of autoantibodies to factor VIII. We report two very different cases. Case 1: This woman experienced onset of juvenile idiopathic arthritis at 8 years of age. Her first child was born when she was 28-years-old. Three months after delivery, vaginal bleeding and recurrent hemarthrosis led to a diagnosis of acquired hemophilia (isolated APTT prolongation, 1% VIIIc activity, and 58 U of anti-factor VIII antibody). Treatment included glucocorticoid therapy, prothrombin complex, and intravenous immunoglobulins. She achieved a full recovery within a year. Case 2: In this 84-year-old woman, spontaneous recurrent hemarthrosis with hematomas revealed idiopathic acquired hemophilia. Treatment included prothrombin complex, factor VIII concentrates, and intravenous immunoglobulins, followed by cyclophosphamide and glucocorticoid therapy. Recovery was complete within a year. The diagnosis, etiology, prognosis, and treatment of acquired hemophilia are discussed. Conclusion: Although rare, acquired hemophilia should be considered among the causes of hemarthrosis, particularly as a favorable outcome can be expected with early diagnosis and appropriate treatment.