Acute mental disorder in early Multiple Sclerosis (MS) is rare and little is known about the structural and metabolic changes in this relation. We present an MS patient with discrete motor and sensory deficits, who developed severe behavioral changes over a period of nine months during the initial course of the disease. The cerebral metabolic rate of glucose (CMRglc) was measured using positron emission tomography (PET), and the patient underwent MRI as well as a comprehensive battery of neuropsychological tests. Significantly reduced values of CMRglc were found bilaterally in the frontal and temporal cortex, the putamen, the thalamus and the hippocampus. The MRI revealed progression of MS lesions in the frontal lobes during the development of mental symptoms. Neuropsychological examination showed wide spread cognitive dysfunction, and a pronounced frontal lobe syndrome. The study demonstrates the remote metabolic effects of lesions affecting subcortical neural connections in an MS patient with severe cognitive dysfunction.