Acquired thrombotic thrombocytopenic purpura (TTP) is caused by autoantibodies against ADAMTS13 that result in severe ADAMTS13 deficiency. TTP usually remits when treated with plasma exchange, however, patients tend to relapse if the autoantibodies persist. Immunosuppression with rituximab can prevent most of these relapses, whether given to all patients at the time of presentation or reserved for disease that is refractory to plasma exchange. The efficacy of rituximab in these settings raises the question of whether patients with asymptomatic severe ADAMTS13 deficiency should be treated preemptively to prevent future relapses. Answering this question depends on understanding both the risk of relapse associated with persistent severe ADAMTS13 deficiency and the risk of adverse events associated with rituximab or other immunosuppressive regimens. Most patients with severe ADAMTS13 deficiency relapse fairly quickly, but some remain asymptomatic for years. Most patients tolerate rituximab well, but rare patients experience serious adverse events that can be fatal, such as progressive multifocal leukoencephalopathy. Some hematologists believe that the data available already justify preemptive treatment to restore ADAMTS13 activity, but others believe that more clinical research is needed before a well-informed decision can be made. Speakers in this session will review the evidence on both sides of the question and consider how to reach a definitive answer.