Objective: To compare the effectiveness of one versus two antibiotics in the treatment of community acquired pneumonia (CAP), related to mortality, inpatient stay and clinical outcomes. Methods: Data from an observational study with CAP was collected during 2005-2007 in Santiago-Chile (sample=321). One group was treated with one antibiotic (n=115) and the other with two antibiotics (n=202). Both were followed during their hospital stay, and clinical outcomes were collected. For each individual, the propensity score was estimated, which is the a priori probability of being assigned to receive treatment with two antibiotics. This was performed with the use of logistic regression, including significant co-variables that occurred before the actual treatment initiation. Analysis of confounder variables was also developed, in order to compare with Propensity Score approach. Results: Univariate analysis showed significant positive association between two antibiotics and the progression to septic shock (RR=4.15), acute renal failure (RR=2.04), acute respiratory failure (RR=3.02) and radiologic impairment (RR=5.10). Combined treatment was also associated with lower probability to be discharged from hospital (HR=0.56). However, when the analysis was adjusted by Propensity Score, these associations disappeared. No inversion of the association was observed. Additionally, when treatment with two antibiotics was adjusted by severity of the CAP (Fine Score), similar results were observed with the exception of acute respiratory failure and hospital stay. Conclusion: In adult patients with CAP, treatment with two antibiotics versus one was not associated with lower mortality, decreased inpatient stay or improved clinical evolution. Analysis using Propensity Score was a useful method, because it was able to explain all those associations observed in the univariate analysis. (Supported by FONDECYT 1050734)