Adenovirus often causes respiratory disease outbreaks in military training soldiers. Compared with adenovirus pneumonia in young military adults, symptoms arising from febrile respiratory illness (FRI) caused by adenovirus have not been previously evaluated in Korean soldiers. We conducted an event-based outbreak investigation involving 712 male soldiers aged 19 to 21 years from March 14 to 30, 2012 to evaluate the epidemiological and clinical characteristics of patients with pneumonia and FRI caused by adenovirus. We described the laboratory and radiological characteristics of patients with adenovirus pneumonia. Among these, 407 cases of FRI and 15 cases of pneumonia were identified through active surveillance (attack rate of FRI, 57.16%; attack rate of pneumonia, 2.11%). Fire training and march training may present environmental risk factors for adenovirus-associated outbreaks. Most symptoms were mild. The most frequent symptom in patients with pneumonia and FRI was cough. Patients with pneumonia were associated with an increased incidence of dizziness (crude odds ratio [cOR], 9.65; 95% confidence interval [CI], 2.38–37.15) and a decreased incidence of rhinorrhea (cOR, 0.15; 95% CI, 0.04–0.53) during adenovirus-associated outbreaks. Differential leukocyte count revealed high monocytes, low lymphocytes, and low eosinophils, and chest computed tomography revealed a consolidation pattern and right lobar pneumonia. These findings warrant a high level of suspicion for adenovirus pneumonia.