An 85-year-old male presented with bilateral chronic subdural hematomas (CSDHs) resulting in unilateral oculomotor nerve paresis and brainstem symptoms immediately after removal of both hematomas in a single operation. Initial computed tomography on admission demonstrated marked thick bilateral hematomas buckling the brain parenchyma with a minimal midline shift. Almost simultaneous removal of the hematomas was performed with the left side was decompressed first with a time difference of at most 2 minutes. However, the patient developed right oculomotor nerve paresis, left hemiparesis, and consciousness disturbance after the operation. The relatively marked increase in pressure on the right side may have caused transient unilateral brain stem compression and herniation of unilateral medial temporal lobe during the short time between the right and left procedures. Another factor was the vulnerability of the oculomotor nerve resulting from posterior replacement of the brain stem and stretching of the oculomotor nerves as seen on sagittal magnetic resonance (MR) images. Axial MR images obtained at the same time demonstrated medial deflection of the distal oculomotor nerve after crossing the posterior cerebral artery, which indicates previous transient compression of the nerve and the brain stem. Gradual and symmetrical decompression without time lag is recommended for the treatment of huge bilateral CSDHs.