Clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) is diagnosed using imaging and is characterized by a favorable prognosis, which often improves without treatment. Here, we report three adult patients with MERS who were treated with a different course each; all of them required immunotherapy to some extent. The first case was a 67-year-old male who experienced gastrointestinal symptoms for 3 months prior to hospitalization. The second was a 38-year-old male who experienced a headache and fever, followed by urinary retention. The third was a 36-year-old male who experienced a fever and headache, followed by dysuria, impaired orientation, and upper limb tremor; he was transported to our hospital on the same day. All patients had lesions in the splenium of the corpus callosum. While two patients responded well to steroid pulse therapy, one demonstrated poor response and required immunoadsorption therapy. While some patients with adult-onset MERS require aggressive immunotherapy, this condition covers a broader range of diseases in adults with varying premonitory and clinical symptoms. Although pediatric MERS is often accompanied with a benign condition, this is not always the case for adults. Thus, considering immunotheraphy may be essential.
