We report a male case of thrombotic microangiopathy with mixed connective tissue disease (MCTD). Thrombocytopenia and hemolytic anemia developed despite steroid treatment for acute interstitial pneumonitis. The patient became confused and eventually developed coma. Diffusion-weighted MRI revealed high intensity at the brainstem, frontal lobes, basal nuclei, and insula, findings compatible with multiple brain edema due to microthrombosis. Despite the treatment of successive plasma exchange and steroid pulse therapy, he eventually died by multiple organ failure. This rare case suggested that diffusion-weighted MRI is very sensitive at detecting early alterations of thrombotic thrombocytopenic purpura (TTP)-induced ischemic lesions in the brain. Neuropsychiatric symptoms due to thrombotic microangiopathy could be some of the fatal complications in MCTD patients.