MRI of 31 patients with dementia of the Alzheimer type (DAT) (mean age 74.7 years) were studied to detect characteristic findings, and compared with those of 24 normal elderly controls (mean age 74.1 years). Atrophy was quantitated by planimetric and linear measurements, and periventricular and deep white matter signal abnormalities were assessed by subjective ratings. Although we observed significant differences between the DAT patients and controls, there was overlap in each of the measurements. Of these, the ratio of the temporal horn area and the pattern of linear measurements (the width of the temporal horn body--the medial temporal lobe width--the interuncal distance) best distinguished the DAT patients from controls. Twenty-five patients (81%) had a ratio of the temporal horn area larger than the value of the mean + 2SD of the controls. AV-shaped pattern, in which the medial temporal lobe width was smaller than the other two values, was demonstrated in 84% of the patients and only 8% of the controls. Although signal abnormalities were not useful for diagnostic purposes, periventricular hyperintensities were more commonly seen in the DAT patients than in the controls, and correlated with cognitive function. MRI studies suggest that the assessment of medial temporal lobe atrophy is useful in the diagnosis of DAT, and periventricular hyperintensity may be related in some way to the disease process.