There are many causes of increased EEG slow waves, which can be seen in traumatic brain injury, intracranial tumors, epilepsy, or intracranial infectious diseases such as encephalitis and meningitis. Also, patients with metabolic encephalopathy with carbon monoxide poisoning and alcoholic encephalopathy can also present with increased slow waves in the brain. In massive cerebral infarction, cerebral hemorrhage, and edema, patients can show increased slow waves on the EEG. Hepatic encephalopathy and pulmonary encephalopathy can also lead to increased slow waves on the EEG. Therefore, the cause of the increased slow waves on the EEG should be determined, and the treatment should be tailored to the cause, with dehydration and drugs to reduce intracranial pressure if necessary. The slow waves caused by epilepsy are accompanied by spike waves, and the patient can be treated with antiepileptic drugs.