Four patients with cerebral hemorrhage were examined serially from the acute to chronic phase by 1H magnetic resonance imaging (MRI), 23Na MRI and computed tomography (CT). At 1-2 days after bleeding, the 23Na image revealed no visible signal change in the area of hemorrhage, although CT and 1H images clearly demonstrated the existence of a hematoma in the thalamus or putamen. At 4-7 days after the hemorrhage, the 23Na images began to exhibit a small increase in signal intensity at the hematoma site, while at 2-3 weeks, a marked increase in 23Na signal intensity was observed. These findings suggest that the hematoma consisted mainly of a corpuscular component, with a low Na+ concentration, with little serum component. Lack of signal from the corpuscular component on the 23Na image was confirmed by an in vitro study. In the late acute phase, Na+ accumulation may occur in the corpuscular component due to failure of the Na+ pump. The intracellular 23Na appears to be totally visible to MRI, resulting in an increase in signal intensity. In the subacute or chronic phase, the corpuscular component may be destroyed, leaving fluid in its place. A high Na+ concentration in this fluid may give markedly increased 23Na signal intensity on MRI. 23Na MRI appears to provide important information for understanding the evolution of cerebral hemorrhage and for estimating the viability of cells, although its value for diagnosis may not be great.