A series of patients referred to the University of Western Ontario, London, Dementia Study for investigation of possible dementia underwent computed tomographic scans, psychometric testing (Extended Scale for Dementia [ESD]), and neurologic examination. Thirty-nine of the 113 patients studied (ischemic score, less than or equal to 4) were found to have leuko-araiosis, which we have defined as patchy or diffuse lucencies in the white matter. Patients with leuko-araiosis had significantly lower mean scores on the ESD, 109.7 +/- 61.2, compared with mean scores of 148.5 +/- 78.0 in those without. However, only a trend toward lower scores on the ESD was observed when age, sex, education, and infarct were taken into account in the analysis of covariance. Leuko-araiosis was found to be associated with increasing age, hypertension, abnormalities of power in the limbs, and extensor-plantar responses in this sample of patients. In patients with Alzheimer's disease (AD) alone, diagnosed clinically, 29 out of 91 demonstrated leuko-araiosis on computed tomography, but scores on the ESD in this group overall were not significantly different when those with and without leuko-araiosis were compared. In less advanced cases, however, a highly significant trend was evident for leuko-araiosis to be associated with increased dementia in AD. The results are consistent with the hypothesis that leuko-araiosis is associated with dementia in AD, and that this is either most marked or most easily identifiable before the dementia becomes very severe.