We reported a rare case of primary interhemispheric subdural abscess. Twenty-three cases of this pathological condition have been reviewed. In those reports, however, findings of magnetic resonance imaging (MRI) were not referred to. In this report, MRI findings in this pathological condition, in addition to X-ray computed tomography (CT), is mentioned. A 22-year-old man, who had suffered from headache and vomiting for 2 weeks, suddenly became drowsy and left-hemiparetic. The X-ray CT scan on admission showed a well-circumscribed low density area in contact with the falx in the right parieto-occipital region. This lesion had so called "ring enhancement". MRI in the sagittal view revealed that, along the falx, the long T1 and T2 areas extended from the right cerebellar tentorium to the right frontal region. The operation demonstrated the capsular formation of the abscess. After pus aspiration, continuous drainage was performed from the cavity of the abscess. The patient fully recovered postoperatively. In the diagnosis of interhemispheric subdural abscess, it is said that conventional X-ray CT sufficiently reveals the quality of the lesion, the precise site, and the anatomical relation to the surrounding edema. In our case, MRI was able to confirm the diagnosis made by the X-ray CT. Furthermore, MRI was thought to be superior to the X-ray CT in the evaluation of the extension of the abscess and in the delineation of the surrounding edema. Combined use of X-ray CT and MRI in cases of interhemispheric abscess was considered to make the diagnosis more precise in both qualitative and quantitative aspects.(ABSTRACT TRUNCATED AT 250 WORDS)