Inherent abnormalities of myelin have been suggested in the pathogenesis of multiple sclerosis (MS). We investigated myelin in acute disseminated encephalomyelitis (ADEM) patients by magnetic resonance spectroscopy (MRS) and cerebrospinal fluid (CSF) analysis for citrulline, a marker of immature myelin. A citrulline peak was observed in the normal appearing white matter of 7/15 patients and of 1/10 age-matched neurological controls (p=0.08). CSF citrulline was elevated in 4/6 patients. Alterations in the composition of myelin might predispose to or follow acute inflammatory disorders of the central nervous system.