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To identify specific risk factors and clinical outcomes of community-acquired pneumonia (CAP) caused by gram-negative bacilli (GNB), we compared the clinical features and outcomes of patients with CAP due to GNB with those of patients with non-GNB pneumonia. We performed a prospective observational study of 912 cases of adult CAP in Asian countries from January 2002 to December 2004. Systemic laboratory evaluation for determining the etiology and clinical evaluation were performed. Of 912 cases with CAP, 93 (10.1%) cases were caused by GNB: 59 with K. pneumoniae, 25 P. aeruginosa, 7 Enterobacter species, 1 Acinetobacter baumannii, and 1 Serratia marcescens. CAP caused by GNB was more frequently associated with septic shock, malignancy, cardiovascular diseases, smoking, hyponatremia, and dyspnea, according to multivariate analysis (P < 0.05). Overall 30-day mortality rate was 7.3% (65/885). Mortality was significantly higher in the GNB group than in the non-GNB group [18.3% (17/93) vs. 6.1% (48/792); P < 0.001]. GNB as a causative microorganism was found to be one of the independent risk factors for mortality (adjusted OR = 2.63, 95% CI 1.02-6.78, P = 0.046) with nursing home residence, mechanical ventilation, cardiovascular disease, respiratory rate > 30/min, and hyponatremia (all P < 0.05). GNB was not only a frequent etiology of severe CAP but also an independent risk factor for mortality. Data suggest that an initial empirical antimicrobial coverage of GNB including P. aeruginosa should be seriously considered in cases of severe pneumonia, especially in patients with underlying malignancy, underlying cardiovascular diseases, smoking, septic shock, and hyponatremia.