Acquired von Willebrand syndrome is a rare bleeding disorder, which has been related in various diseases including lymphoproliferative disorders or autoimmune diseases. Its diagnosis is an important step before treatment of patients and particularly in case of bleeding. We report four cases from Caen Hemophilia Treatment Center, diagnosed and treated from 1999 to 2008. Mucocutaneous bleeds in every case were the same as in hereditary von Willebrand disease. All patients had no personal or family history of bleeding. Phenotype was identified as type 2 von Willebrand disease with a loss of high molecular weight multimers. Anti-von Willebrand factor inhibitor screening was positive for three patients. The etiological diagnosis was one chronic lymphocytic leukaemia, two monoclonal gammapathies of undetermined significance (MGUS) and one undetermined case. The management of patients need two stages: first infusions of factor von Willebrand/factor VIII concentrates to stop bleeds, then treatment of the underlying disease such as chemotherapy, corticotherapy and treatment with high doses of polyvalents immunoglobulins. In every case, treatment was effective and improved patient's quality of life.