Patients with hypercoagulability may present with a single thrombosis and subsequently develop progressive thromboses at other sites. With inadequate therapy, the thrombotic process may self-perpetuate, leading to multiple thromboses and even death. Six cases are presented demonstrating key features of what may be termed thrombotic storm: (1) an underlying hypercoagulable disorder; (2) a provocation to initiate thrombosis; (3) rapid development of new thromboses; (4) response to prompt use of thrombolytic agent or anticoagulant therapy; and (5) remarkable good long-term prognosis if the cycle of thrombosis is interrupted. Continued activation of coagulation by fresh thrombosis is hypothesized as the cause of the syndrome, which may explain its control by anticoagulants. Whereas these unusual patients' courses most likely represent only an extreme of hypercoagulability and not a new disorder, their characteristic behavior warrants attention.