Carbon monoxide (CO) poisoning leads to demyelination of cerebral white matter (CWM) fibers, causing chronic neuropsychiatric symptoms. To clarify whether fractional anisotropy (FA) from diffusion tensor imaging in the centrum semiovale can depict demyelination in the CWM during the subacute phase after CO inhalation, we examined correlations between FA in the centrum semiovale and myelin basic protein (MBP) in cerebrospinal fluid. Subjects comprised 26 adult CO-poisoned patients ≤60 years old. MBP concentration was examined for all patients at 2 weeks after CO inhalation. The mean FA of the centrum semiovale bilaterally at 2 weeks was also examined for all patients and 21 age-matched healthy volunteers as controls. After these examinations, the presence of chronic symptoms was checked at 6 weeks after CO poisoning. Seven patients displayed chronic symptoms, of whom six showed abnormal MBP concentrations. The remaining 19 patients presented no chronic symptoms and no abnormal MBP concentrations, with MBP concentrations undetectable in 16 patients. The MBP concentration differed significantly between patients with and without chronic symptoms. The mean FA was significantly lower in patients displaying chronic symptoms than in either patients without chronic symptoms or controls. After excluding the 16 patients with undetectable MBP concentrations, a significant correlation was identified between MBP concentration and FA in ten patients. The present results suggest that FA in the centrum semiovale offers a quantitative indicator of the extent of demyelination in damaged CWM during the subacute phase in CO-poisoned patients.