The diagnosis and treatment of thrombotic thrombocytopenic purpura (TTP) in patients following BMT are often uncertain and unsuccessful. To better understand the evaluation and management of these patients, we describe 17 patients treated with plasma exchange for a presumptive diagnosis of TTP following BMT during a 10 year period, 1989-1998. Because of the uncertainty of the diagnosis, these patients are described as having a 'TTP-like syndrome'. All 17 patients had received an allogeneic BMT. Comparison with the other 245 patients who had an allogeneic BMT during the same period demonstrated that patients with a TTP-like syndrome more frequently had unrelated and/or HLA-mismatched donors, and had also experienced more serious complications: grade III-IV acute GVHD and systemic bacterial, fungal, and viral infections. Three months after the diagnosis of the TTP-like syndrome, only four of 17 patients (24%) were alive; currently only one patient survives. These data emphasize: (1) the diagnosis of TTP following BMT is uncertain because of the presence of multiple BMT-associated complications. (2) The outcome of patients with TTP-like syndromes following BMT is poor. (3) Urgent intervention with plasma exchange when TTP is suspected following BMT may not always be appropriate. Alternative explanations for the signs and symptoms should be considered and treated aggressively.