Background: Pneumonia is one of the most common complications of influenza infection. However, its characteristics are not well-known. Our aim was to assess the frequency of community-acquired pneumonia (CAP) associated with influenza virus and to determine its clinical and epidemiological characteristics., Patients and Method: A total of 240 patients with CAP were evaluated and cases with serological evidence of influenza virus infection were identified. Patients in whom there was no evidence of infection by other microbial agents were selected and defined as influenza virus infection-associated CAP (CAP-i). Clinical and epidemiological features of CAP-i patients were compared with the rest of patients with CAP., Results: A microbial diagnosis was achieved in 143 (59.6%) of 240 patients with CAP. In 12 cases (5%) there was serological evidence of infection by influenza A virus. In 8 patients no other respiratory pathogens were detected (CAP-i). In a multivariate analysis, the presence of arthromyalgias (odds ratio [OR] = 30, 68; 95% confidence interval [CI], 1-946), plasmatic sodium < 133 mEq/l (OR = 16.92; 95% CI, 1.48-193), bilateral infiltrates in chest X-ray (OR = 8.83; 95% CI, 1.35-57.6), diagnosis of CAP in January or February (OR = 5.60; 95% CI, 0.87-35.8) and blood neutrophil cell count >= 14,000/l, (OR = 5.23; 95% CI, 0.85-32) were all independently associated with CAP-i. The simultaneous presence of all these characteristics had a positive and negative predictive value of 95% and 99%, respectively, to differentiate CAP-i from other CAP., Conclusions: Influenza virus infection-associated CAP is uncommon. Some clinical and radiographic variables could differentiate CAP-i from other CAP. These parameters could also help identify patients who are more likely to benefit from specific antiviral therapy.