Four methods of measuring the suprasellar cistern (SSC), the SSC area ratio, the hexagonal area ratio, the width ratio, and perceptual rating, are described. All four are found to be reliable across raters, time, and procedure (light box vs. computer), and all are able to distinguish Alzheimer's Disease (AD) and Huntington's Disease (HD) patients from their respective age-matched controls. The width ratio, a measure of minimum distance between the two temporal lobes divided by brain width at the same level, was most effective in distinguishing HD from AD patients when age was used as a covariate. Because the width of the SSC measures separation of the temporal lobes at the level of the amygdala and entorhinal cortex, it is reasonable to expect this measure to distinguish between AD and HD, as the amygdala and entorhinal cortex are known to be particularly severely affected in AD, but not in HD, patients. The other measures of SSC were most highly correlated with cognitive status in AD patients. Several possible explanations for this finding are offered. No SSC measure was significantly correlated with cognitive status in HD. This suggests that atrophies of the frontal-temporal and basal forebrain areas are associated with cognitive decline in Alzheimer's Disease but not in Huntington's Disease.