Here, we present a case of a severe acute disseminated encephalomyelitis (ADEM) of a 42-year-old male patient. The diagnosis was established after brain biopsy and due to acutely evolving encephalopathy occurring in the context of atypical Mycoplasma pneumoniae (MP). We analysed the prominent MRI white matter lesions using a three-dimensional algorithm as cutting-edge technique to study morphological abnormalities and correlated them to the clinical condition of the patient. We found a discrepancy between the lesion size and the clinical deficits of the patient, actually the clinical improvement antedated the regression of the white matter lesions.