We investigated whether hyponatremia is a risk factor of death in patients hospitalized with community-acquired pneumonia (CAP) and estimated the relative risk of death by CAP of other risk factors. The design was prospective multicentre cohort study. In 5 centers in Buenos Aires, Argentina, we studied adults hospitalized with CAP between March 21, 2000 and December 21, 2000. Using stepwise logistic regression, we analyzed risk factors that showed a univariate association with mortality; alpha significance level was 0.05. During a 9-month period, 238 patients were admitted with CAP: 150 (63%) male and 88 (36%) female, mean age 52.99 (+/-20.35) and 55.06 (+/-20.94), respectively. Mortality was 10.5% (25/238). By multivariate analysis, the following variables were statistically associated with evolution: cerebrovascular disease (CD) (B: 2.614, p < 0.001, RRE: 13.6, IC 95%: 3.7-49.6); hyponatremia at admission or during hospitalization (B: 1.994, p<0.001, RRE: 7.3, IC 95%: 2.5-20.8); and elevated blood urea (B: 0.016, p= 0.003, RRE: 1.016, IC 95%: 1.005-1.02). We developed a formula to predict mortality by CAP: P (death) = 1/1 + exp - (-4.03 + 2.61 x l + 1.99 x 2 + 0.016x3), where: x1=CD (yes = 1/ no=0); x2= hyponatremia (yes = 1/ no=0); x3 = blood urea (mg/dl). The predictability was 91.1%. The mortality risk by CAP was statistically higher in patients with CD, hyponatremia and elevated blood urea.