In July 2009, a pandemic influenza (H1N1) (pdm H1N1) virus epidemic emerged rapidly in Phitsanulok, Thailand. Adult cases of community-acquired pneumonia (CAP) were prospectively examined for pdm H1N1 virus infections by real-time PCR in a tertiary hospital in Phitsanulok from July to November 2009. Twenty-four cases of pdm H1N1 virus-associated CAP were confirmed, and their clinical features including bacterial infection, severity of disease, course of admission, treatment, and outcome were investigated. The median age of these cases was 39.5 years. Most cases appeared to be primary viral pneumonia, but only one case was positive for a urinary pneumococcal antigen. The median time from the onset of illness to admission was 4 days. All 24 patients received oseltamivir after admission. Twelve (50.0%) were defined as having severe CAP and 9 (37.5%) were diagnosed with acute respiratory distress syndrome (ARDS). During the study period, pdm H1N1 virus infections frequently caused severe CAP among young adults because of the delayed initiation of antiviral therapy. Of the 9 ARDS patients, 3 died of ventilator-associated pneumonia caused by multidrug-resistant Acinetobacter baumannii. Implementation of infection control targeting this pathogen is required in tertiary hospitals in Thailand.