The purpose of the present study is to assess the efficacy of methylprednisolone as an adjuvant therapy in patients with severe community-acquired pneumonia (CAP) (PSI 4-5).

The hypothesis of the study is that methylprednisolone can decrease the mortality of severe CAP without any significant side effects,with reduction of the time to clinical stability and failure rate of treatment.

Inclusion Criteria:

• community-acquired pneumonia
• PSI score：4-5
• with at least one of following conditions：①CRP ≥150 mg/L,②oxygen index ≤250 , ③bilateral or multi-lober infiltrates in chest image,④ area of pulmonary infiltrates increase >50% with 48 hous,⑤fever higher than 39℃ exceed 72 hours

Exclusion Criteria:

• nosocomial Pneumonia
• aspiration pneumonia
• acute burn injury
• gastrointestinal bleeding within the past three months
• uncontrolled diabetes mellitus
• pregnant or breast feeding
• a condition requiring treatment with greater than 0.5 mg/kg/d of prednison or its equivalent
• severe immunosuppression (human immunodeficiency virus infection, immunosuppressive conditions or medications)
• active tuberculosis
• preexisting medical condition with a life expectancy of less than 3 months