Cerebral swelling after cardiopulmonary bypass might trigger a critical cerebral consequence resulting from intracranial space-occupying lesion. We experienced a 75-year-old woman who suffered from a delayed left hemiplegia after mitral valve replacement. Urgent diagnostic imaging revealed the presence of a brain tumor with perifocal cerebral edema. Fluid shifts occurring within a few days after the cardiopulmonary bypass, manifesting the focal cerebral edema, played a key role in this unique clinical course.