There are several new magnetic resonance (MR) technologies, which will provide data about the pathogenesis, circulatory state and cellular effects of ischemic stroke. Among them, diffusion MRI is the most promising one. This technology clearly reveals, within minutes of stroke onset, ischemic lesions which ultimately progress to infarct. The early ischemic lesions demonstrated by diffusion MRI represent regions with slower water diffusions perhaps due to intracellular water accumulation or shrinkage of the extracellular spaces resulting from ionic pump failure, spreading depression, or other mechanisms. The diffusion MRI can show not only irreversible but also reversible ischemic lesions, and therefore has a potential to discriminate salvageable tissues from irreversibly damaged tissues before a therapeutic intervention with thrombolytic and/or cytoprotective agents. The diffusion MRI can highlight only a fresh lesion, because water diffusion is decreased during several days after stroke onset. Diffusion MRI is extremely prone to motion artifacts, a major concern against clinical application. This problem can be solved by combination with an ultrafast technique, echo planar (EP) imaging. The EP diffusion MRI will become a rapid, reliable, objective and essential emergency diagnostic test in a clinical setting that will guide the development and application of acute therapeutic intervention.