Background and Purpose. To study whether computed tomography (CT) can measure the water content of early ischemic edema. Methods. The authors obtained cranial CT in 5 groups of rats subjected to 1 hour (n = 8), 2 hours (n = 11), 3 hours (n = 13), 4 hours (n = 13), or 6 hours (n = 14) of right middle cerebral artery (MCA) occlusion. Immediately after CT, the authors removed the rats' brains and determined tissue water content by the dry-wet weight method. They correlated brain x-ray attenuation with brain tissue water content. Results. Mean brain tissue water content remained constant in the nonischemic left hemispheres at 77.9% ± 0.6% and increased up to 79.3% ± 1.0% in the right hemispheres after 6 hours of permanent right MCA occlusion. X-ray attenuation remained constant in the left hemispheres at 75.6 ± 2.2 Hounsfield units (HU) and decreased to 71.7 ± 3.4 HU in the right hemispheres after 6 hours of right MCA occlusion. The decrease in x-ray attenuation correlated significantly with the increase in ischemic brain tissue water content (y = 217.3 - 1.8 × x; r = .55, P < .0001). That means that a 1% increase in hemispheric tissue water content causes a decrease in x-ray attenuation of 1.8 HU. Conclusions. After MCA occlusion, immediate brain tissue net water uptake is associated with a decrease in x-ray attenuation, CT can monitor ischemic edema in an acute stroke.