We report the case of a 15-year-old boy with clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS). He presented with diplopia and ataxia and his cerebrospinal fluid showed mild pleocytosis and elevated protein. MRI demonstrated a reversible splenial lesion in the corpus callosum. He did not receive any treatment and recovered quickly within two weeks. Results of neurological examination after 3 months were completely normal. MERS is relatively unknown in Europe. The majority of patients are reported in East Asia. This post-infectious encephalitis/encephalopathy arises soon after the onset of symptoms. The prognosis is excellent and most patients recover completely without neurological sequelae. MRI typically shows a reversible splenial lesion with diffusion restriction and without contrast enhancement, sometimes with adjacent symmetrical lesions extending into the subcortical white matter. The pathogenesis is still unknown. Recognition of the condition and its clinic-radiological discrimination from acute disseminated encephalomyelitis may prevent unnecessary treatment.