Brain tumoral microembolism is a very unusual etiology of cerebral vascular disease in patients with cancer. The clinical course is characterized by diffuse encefalopathy with associated neurologic deficits. We report a 72-year-old male patient, who developed rapidly progressive dementia associated with diffuse myoclonic jerks, without neurologic deficit, whose brain biopsy showed microscopic emboli of adenocarcinoma of unknown origin in capillary vessels. We did not find metastasis in other organs. We suggest that this entity should be included in the differential diagnoses of the subacute dementias.