A newly developed free radical scavenger, 3-methyl-1-phenyl-2-pyrazolin-5-one (MCI-186), holds promise for clinical application. We clinically evaluated the effect of MCI-186 on cerebral infarction by using magnetic resonance imaging (MRI) and proton MR spectroscopy (MRS). Six patients with large supratentorial infarction were evaluated with sequential MRI and proton MRS. These patients were also administered MCI-186 for 14 days after ischemic insult (MCI-186 group). The findings were compared with those for patients who had supratentorial infarctions equivalent in size to those in the MCI-186 group but who had received only conventional therapy. The course of change of the size of infarction was evaluated by MRI, and the metabolic changes following cerebral infarction were evaluated by proton MRS. As a result, there was no significant difference between the initial size of infarction in the conventionally treated group and that in the MCI-186 treated groups, nor did the groups show significant difference in the sequential changes depicted by MRI in the area of infarction, midline shift, or amount of edema. However, on MRS, the N-acetyl aspartate signal was significantly higher in the MCI-186 group than in the conventionally treated patients. In conclusion, MCI-186 has an effect of preservation of N-acetyl-aspartate, which is thought to be a neuronal marker, in cerebral infarction.