Fifty-six patients diagnosed with definite multiple sclerosis (MS) according to Poser criteria were administered the Mini-Mental State Examination (MMSE) and a comprehensive battery of neuropsychological tests. Extent of cerebral lesion involvement was determined by quantitative magnetic resonance imaging (MRI) ratings. The MMSE correlated with overall levels of physical disability, but did not correlate with total lesion area on MRI. Sensitivity of the MMSE to the sub cortical dementia of MS was low (28%) when performance on the neuropsychological testing battery was used as the criterion. Impairment on tests of memory, speed of information processing, abstract reasoning, naming/verbal fluency, as well as visuoperceptual organization, were correlated highly with total lesion area on MRI. The low sensitivity of the MMSE to cognitive impairment in MS is discussed in terms of its item composition and the characteristic pattern of deficits found in MS.