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Background. The role of viral infections in the etiology of severe community-acquired pneumonia (SCAP) was prospectively evaluated from 2008 to 2012 at a university-level intensive care unit. Methods. Clinical data and microbiological tests were assessed: blood cultures, urine pneumococcal and legionella antigens, Mycoplasma pneumoniae and Chlamydia pneumoniae antibodies from paired serums, and respiratory virus detection by multiplex, real-time polymerase chain reaction (PCR) from nasopharyngeal swabs and lower tracheal specimens via intubation tube. Results. Of 49 mechanically ventilated SCAP patients (21 men and 28 women; median age, 54 years), the etiology was identified in 45 cases (92%). There were 21 pure bacterial infections (43%), 5 probably pure viral infections (10%), and 19 mixed bacterial–viral infections (39%), resulting in viral etiology in 24 patients (49%). Of 26 viruses, 21 (81%) were detected from bronchial specimens and 5 (19%) from nasopharyngeal swabs. Rhinovirus (15 cases, 58%) and adenovirus (4 cases, 15%) were the most common viral findings. The bacterial–viral etiology group had the highest peak C-reactive protein levels (median, 356 [25th–75th percentiles, 294–416], P = .05), whereas patients with probably viral etiology had the lowest peak procalcitonin levels (1.7 [25th–75th percentiles, 1.6–1.7]). The clinical characteristics of pure bacterial and mixed bacterial–viral etiologies were comparable. Hospital stay was longest among the bacterial group (17 vs 14 days; P = .02). Conclusions. Viral findings were demonstrated in almost half of the SCAP patients. Clinical characteristics were similar between the pure bacterial and mixed bacterial–viral infections groups. The frequency of viral detection depends on the availability of PCR techniques and lower respiratory specimens.