To examine the relationship between cancer and development of thrombotic microangiopathy (TM), the medical records of patients with known TM were examined in one institution from January 1981 to December 2002. Nine out of 93 patients with the established diagnosis of TM had active cancer. All nine of those patients had thrombotic thrombocytopenic purpura (TTP). Among those patients, two patients received chemotherapy prior to the development of TTP. Six of the seven patients who received no chemotherapy had extensive bone marrow metastasis and secondary myelofibrosis. There were two patients each with breast cancer, lung cancer, and stomach cancer. Severe anemia and thrombocytopenia with leukoerythroblastosis were prominent clinical features in all six patients. Four patients had neurological (mental) changes and three developed fever, but none had significant renal dysfunction. Upon establishing the diagnosis of TTP, four patients were treated with exchange plasmapheresis (EP) and two patients were treated with chemotherapy because there were no neurological changes. Three patients achieved complete remission of TTP, one with EP alone and two with chemotherapy. The one patient who achieved remission with EP alone was later treated with chemotherapy and survived for 2 1/2 years. The other three patients treated with EP alone died within 2 months after the diagnosis of TTP. Since TTP occurred in association with bone marrow metastasis and myelofibrosis in six patients among seven chemotherapy-untreated cancer patients, this marrow change was considered to be the possible cause of the development of TTP. It is recommended that all cancer patients with unexplained anemia and thrombocytopenia be evaluated for the coexistence of bone marrow metastasis and TTP.