The patient was a 70-year-old man. In December 2014, he was diagnosed with dermatomyositis (anti-TIF1-gamma-antibody positive). Pulsed mPSL (1,000 mg/day for 3 days) was commenced, and PSL (60 mg/day; 1.2 mg/kg/day) combined with IVIG (20 g/day; 400 mg/kg/day for 5 days) was also administered. In February 2015, gastric cancer was identified, and laparoscopic distal gastrectomy was performed. In March 2015, the dermatomyositis relapsed (both skin symptoms and muscle weakness were evident), so the PSL dose was increased to 30 mg/day, combined with IVIG 20 g/day (400 mg/kg/day for 5 days). However, the effects were inadequate. Thus, in April 2015, pulsed mPSL (1,000 mg/day for 3 days) was commenced, and the PSL dose was increased to 70 mg/day (intravenously). Left pleural effusion was noted, and Mycobacterium tuberculosis was identified on culture. He was transferred to our hospital for further treatment. On day 8 of hospitalization, sudden thrombocytopenia was evident (platelets 3,000 /muL). The other laboratory findings were as follows: Hb level, 7.2 g/dL and WBC count, 4,030 /muL. A peripheral blood film revealed fragmented erythrocytes. The direct Coomb's test was negative, and the normal coagulation profile rendered DIC unlikely. The relevant biochemical findings included a markedly reduced serum haptoglobin level (2 mg/dL) and an elevated serum lactate dehydrogenase (LDH) level (442 U/L; reference range, 119-229 U/L). Furthermore, PBSs revealed many fragmented red blood cells. The laboratory data were consistent with TMA. We stopped all drugs except for PSL in order to exclude the possibility of drug-induced thrombocytopenia and commenced plasma exchange. No ADAMTS13 activity was detectable, and anti-ADAMTS13 IgG autoantibodies were evident. In addition, many von Willebrand multimers were apparent in the blood. We therefore diagnosed him with TTP. We continued daily plasma exchange, but the thrombocytopenia continued. Altered consciousness was evident on day 17 of hospitalization, and convulsions developed on day 18. Therefore, we added rituximab on day 19 and pulsed mPSL (1,000 mg/day for 3 days) and IVIG (17.5 g/day) on day 22. The PSL dose was increased to 70 mg/day. On day 44, the ADAMTS13 activity increased to 14.7%, and the anti-ADAMTS13 autoantibody status was negative. In addition, the haptoglobin level continued to increase. Therefore, plasma exchange was stopped. However, the thrombocytopenia remained (10,000 platelets/muL), and the condition progressed to pancytopenia. Renal dysfunction also emerged (serum creatinine increased from 0.5 to 1.3 mg/dL). The patient was diagnosed with myelodysplastic syndrome (MDS) based on a bone marrow biopsy performed on day 121 of hospitalization. On day 101, bile duct inflammation and septic shock attributable to Enterobacter cloacae were apparent. He responded to both antibiotics and endoscopic retrograde cholangiopancreatography (ERCP). However, on day 143, he developed septic shock attributable to multi-drug-resistant Pseudomonas aeruginosa (MDRP) and died on day 158.