BACKGROUND AND PURPOSE: Influenza is an important cause of acute respiratory illness among military recruits, and pneumonia is the most frequent complication. This study was performed to categorize the clinical manifestations of influenza infections among military recruits. METHODS: In this retrospective chart review, epidemiologic investigation of patients who met the definitions of acute respiratory illness, influenza-like illness, and pneumonia was conducted. Surveillance of influenza by viral culture and polymerase chain reaction was performed weekly from a random selection of 4 patients with influenza-like illness of less than 3 days duration. RESULTS: 2074 and 2046 men recruited to the Substitute Service Training Center in Taiwan, from November 30 to December 31, 2006 (outbreak 1), and January 11 to February 12, 2007 (outbreak 2), respectively were enrolled. During outbreak 2, 1182 men (57.7%) were identified to have acute respiratory illness, including 607 (29.6%) with influenza-like illness and 19 (0.9%) with pneumonia. During outbreaks 1 and 2, sixty two nasal and throat swabs were obtained, 15 of which were influenza A and 6 were influenza B. All the influenza A isolates were A/Wisconsin/67(H3N2) viruses. Haemophilus influenzae was isolated from 9 of 19 patients with pneumonia (47.3%); 8 from sputum specimens and 1 from a blood specimen. H. influenzae was the primary identifiable bacterium. CONCLUSIONS: The 2 outbreaks consisted of concurrent infection of influenza A and B, with subsequent pneumonia. These results have implications for outbreak management and treatment of influenza among military recruits. Surveillance of influenza-like illness enables early detection of an outbreak and better understanding of the epidemiology of influenza in this setting.