The patient was a 62-year-old man who developed severe antibiotic-refractory pneumonia during the treatment of rheumatoid arthritis with adalimumab. Emergent intratracheal intubation was conducted because of severe respiratory failure he had on admission. The platelet count was found to be markedly decreased without abnormality of hemostasis. Legionella' pneumonia was diagnosed based on the observation of bacilli in Gimenez-stained specimens of aspirated sputum. Plasmapheresis was performed when a diagnosis of thrombotic thrombocytopenic purpura (TTP) was made on the basis of the five classic signs. He tended to get better temporarily, but died from such complications as tension pneumothorax. Severe infection can be associated with secondary TTP, which should be differentiated from disseminated intravascular coagulation because TTP needs to be promptly treated with plasmapheresis.