SUMMARYThe possibility that the brain scan can be used to estimate the magnitude of a cerebral lesion is tested by correlating the volume of a lesion calculated from its scan image with [1] a functional deficit in simple reaction time and [2] its volume measured from pathological data. In 20 patients with unilateral lesions of the right hemisphere, significant correlations were obtained with both auditory and visual reaction times. For cases of primary tumor, the correlations were highest and indicate that reaction time is elevated in direct proportion to lesion volume. An error of about 30% in estimating actual lesion volume from the scan is indicated. For metastases and vascular lesions, the correlations between scan volume and reaction time were positive but not statistically significant. In 23 cases with lesions of the left hemisphere, on the other hand, reaction time and scan volume were not correlated, although reaction times were higher than for normal subjects. In 7 cases of primary tumor that came to autopsy, a correlation of +.87 was obtained between lesion volume estimated from the brain scan and from pathological data.