Objective: To establish the etiology of Community Acquired Pneumonia (CAP) in adult immunocompetent patients consulting to the Emergency Room at Clinica Reina Sofia. Material and Methods: 138 patients (p) with CAP that met inclusion criteria and signed informed consent were included. All the patients underwent all the following: clinical examination, chest x-ray, CBC, Sputum samples for Gram, Ziehl Nielsen, KOH and Pneumocystis colorations; Sputum and blood cultures, acute phase serum for Legionella pneumophila S1 IgM antibodies, Mycoplasma pneumoniae, Coxiella burnetti, Chlamydia pneumoniae, Adenovirus, RSV, Influenza A and B, parainfluenza 1, 2, 3; Nasopharyngeal swabs for viral antigens and Urinary Antigen for Legionella pneumophila and S. pneumoniae. Results: From 138 patients, 74 were females (53.6%) and 64 (46.4%) males. They were between 18 and 93-year-old with an average of 61± 19 years, most patients, 74 (53,62%) were over 65 years old. 51 patients (36.95%) had COPD, twelve patients were smokers (8.7%), 72 nonsmokers (52.2%), 48 past smokers (34.8%) and 6 cooked with wood (3.62%). 37patients (26.81%) had vaccination against influenza, 19 patients (13.8%) had pneumococcal vaccination. The average time of evolution was 5.48 ± 3.3 days. The most frequent symptoms were fever (97.82%), cough (83.3%), and shortness of breath (78.98%). Etiological diagnosis was established in 64 patients (46.4%), 12 of them had two germs isolated, for a total of 75 isolates. The first five isolates as single germ, were the following: S. pneumoniae 22 (16%), H. parainfluenzae 5 (3.62%), L. pneumophila 4 (2.9%), S. aureus 3 (2.17%) and H. influenzae 2 (1.45%). Summed together, viruses were responsible for six cases, 5 as single germ and in one case two viruses in association. Viral etiology was the third cause in this series. The blood cultures were positive in 4 (2.9%) patients. Conclusions: the most frequent isolate, both as single or mixed aetiology was S.pneumoniae, followed by atypical germs and viruses. Of importance, we found a low performance of the blood cultures and a high number of L. pneumophila isolates, which must be taken into account to start the first antibiotic therapy