We studied five patients with herpes simplex encephalitis type 1 with diffusion-weighted MR imaging, as this sequence provides information regarding tissue integrity. Diffusion-weighted imaging was obtained using the echo-planar sequence. True diffusion images (i.e. b=1000s/mm(2) images), and the ADC (apparent diffusion coefficient) values were studied. In addition, diffusion imaging of 12 normal cases, 46 patients with acute ischemia (cytotoxic edema), and 12 patients with vasogenic edema were studied for comparison. In patients with herpes encephalitis, two distinct types of diffusion imaging findings (on b=1000s/mm(2) images, and ADC maps) were noted: lesions similar to cytotoxic edema, and lesions similar to vasogenic edema. The patients with the former type of lesions had fulminating disease, and were in severe clinical condition. Those with the latter represented early cases, and they were in fairly good clinical condition with a good outcome by prompt therapy. Diffusion imaging appears to be a promising sequence to monitor the changes in the brain tissue in herpes encephalitis, and in other infections as well with respect to restriction of movement (cytotoxic edema) or relatively high-motion (vasogenic edema) of water molecules, providing data on the severity of the disease.