Weakness in the right leg is a sign of limb weakness, which may be caused by cerebrovascular disease and can also be seen in patients with encephalitis and brain tumors. Cerebrovascular diseases usually have an acute onset and include cerebral hemorrhage and cerebral infarction, mostly seen in lesions on the left side of the brain. Cerebral hemorrhage usually develops during activity, and most have a previous history of hypertension. In the case of cerebral infarction, the patient has a quiet onset and most patients have diabetes mellitus, hyperlipidemia, cerebral atherosclerosis, and hyperhomocysteinemia, and should be examined with cranial CT or cranial MRI to clarify the diagnosis. It is also important to check blood lipids, blood sugar, blood pressure, and cranial MRI if necessary to clarify the possibility of aneurysm and arteriovenous malformation, and its treatment varies depending on the disease causing it. In the case of encephalitis, the patient has a subacute onset and can have fever and headache at the same time, and electroencephalography and cerebrospinal fluid examination can clarify the diagnosis. In the case of brain tumor, the patient has a chronic onset and cranial MRI can confirm the diagnosis.