Pneumonia is a major infectious cause of death worldwide and imposes a considerable burden on healthcare resources. Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid and involved in many physiological processes including immune responses and endothelial barrier integrity. In term of endothelial barrier integrity, S1P plays a crucial role in protecting lungs from pulmonary leak and lung injury. Because of the involvement in lung injury, S1P could be the potential biomarker of pneumonia. Recently, our pilot study suggested that patients with CAP have significantly higher plasma S1P levels than healthy individuals. Interestingly, our observational study also showed significantly elevated S1P level in the patients who were treated with methylprednisolone during the hospitalization. Based on the above evidence, we hypothesize that S1P plays an important role in the pathobiology of pneumonia. Moreover, S1P is not only a useful biomarker for diagnosis of CAP, but also can be an indicator for using corticosteroids adjuvant therapy.