Background Severe sepsis, may be present on hospital arrival in approximately one-third of patients with community-acquired pneumonia (CAP). Objective To determine the host characteristics and micro-organisms associated with severe sepsis in patients hospitalized with CAP. Results We performed a prospective multicenter cohort study in 13 Spanish hospital, on 4070 hospi- talized CAP patients, 1529 of whom (37.6%) presented with severe sepsis. Severe sepsis CAP was independently associated with older age ( > 65 years), alcohol abuse (OR, 1.31; 95% CI, 1.07 - 1.61), chronic obstructive pulmonary disease (COPD) (OR, 1.75; 95% CI, 1.50 - 2.04) and renal disease (OR, 1.57; 95% CI, 1.21 - 2.03), whereas prior antibiotic treat- ment was a protective factor (OR, 0.62; 95% CI, 0.52 - 0.73). Bacteremia (OR, 1.37; 95% CI, 1.05 - 1.79), S pneumoniae (OR, 1.59; 95% CI, 1.31 - 1.95) and mixed microbial etiology (OR, 1.65; 95% CI, 1.10 - 2.49) were associated with severe sepsis CAP. Conclusions CAP patients with COPD, renal disease and alcohol abuse, as well as those with CAP due to S pneumonia or mixed micro-organisms are more likely to present to the hospital with severe sepsis.