Infectious mononucleosis (IM) is a benign disease and is frequently associated with mild respiratory symptoms. The pulmonary manifestations rarely dominate the clinical picture. We report a case of an older adult patient with IM who developed diffuse pneumonia and acute respiratory failure. The diagnosis of IM was confirmed serologically, whereas other possible causes of pneumonia were excluded in this study. Pulmonary manifestations of IM are discussed, and the atypical features that may be seen in older adults are simultaneously reviewed.