In March 2009, a novel influenza H1N1 virus (2009 H1N1) emerged and spread worldwide. We describe the clinical course of pneumonia in patients hospitalized with 2009 H1N1 in Korea from August to mid-October 2009. Of 351 patients with a confirmed 2009 H1N1 infection, a total of 17 patients with pneumonia were admitted to an acute care unit. More than half of the patients were between 7 and 23 y of age, and only 2 patients were 65 y of age or older. Only 6 patients (35.3%) had underlying medical conditions. For 10 of the 17 patients, chest radiographs taken on admission had findings of pneumonia with bilateral infiltrates. Despite intensive resuscitative efforts, 1 adult patient arriving with severe hypoxemia in the emergency department died 3 weeks later. However, we were able to discharge most of the patients without any other complication in 5 days. 2009 H1N1 was the cause of pneumonia with mild hypoxemia at admission in previously healthy school-aged persons in Korea.