Background: Differentiated diagnosis of meningoencephalitis in elderly patients, when an uncommon picture of respiratory infection leading to acute hypercapnic respiratory failure (AHRF) and without neurological signs is present, may raise serious difficulties. Case report: Two patients aged 72 (patient 1) and 75 (patient 2), without any medical history of respiratory problems, were hospitalized during spring with influenza symptoms. Within less than 24 hours from their admission, patients developed AHFR, which led to disturbances of arterial blood gases (ABGs). They were intubated and transferred, mechanically ventilated, to our intensive care unit (ICU). After normalization of ABGs, a weaning process followed. Weaning was unsuccessful, as unexpected neurological semiotics occurred. The imaging of neurological MRI revealed no special damage apart from microangiopathy. Further investigations through lumbar puncture with cerebrospinal fluid (CSF) testing revealed meningoencephalitis. Antiviral treatment was applied and the outcome was successful for both patients. Conclusion: Critical care medical personnel should be aware of the event of viral meningoencephalitis in elderly patients with AHRF. Especially in the middle of influenza endemic periods, the management of elderly patients may not be appropriate, thus resulting in serious delays for a proper treatment of disease. Laboratory screening of blood and urine, as well as other body fluids, can help detect and determine brain infection. Results from these tests can help exclude other diseases that mimic meningoencephalitis. In cases of patients who are hospitalized in the ICU due to AHRF and do not exhibit neurological abnormalities from the onset of illness, lumbar puncture and check of CSF should be a routine examination. Prophylactic vaccination of high-risk groups may be of crucial importance for preventing complications in patients who develop meningoencephalitis. However, once infected with this disease, the positive outcome is highly dependent on the immediate diagnosis and proper treatment of patients in the ICU.