This work is concerned with the accurate quantification of brain water content under routine clinical conditions. Gelatin solutions of varying water content are first employed as a model of an edematous brain and longitudinal relaxation measurements are performed at proton Larmor frequencies of 5, 41, 63, and 100 MHz. These are followed with in vivo measurements in an experimental animal model of brain edema at 41 MHz. The results underscore the dominant role of total water content W in the relaxation process and verify the expected linearity between 1/T1 and 1/W. A scheme is presented and experimentally verified at 41 MHz for deducing the exact relationship of 1/T1 vs 1/W at any frequency. Knowledge of this relationship along with precise measurements of 1/T1 at a given field strength permits quantitative in vivo measures of brain water content to be obtained with a precision of about 0.01. It is concluded that routine, accurate, and noninvasive brain water measurements are possible by magnetic resonance imaging in a clinical environment.