The purpose of this study was to demonstrate the prevalence of and risk factors for pneumonia after cytotoxic chemotherapy in advanced lung cancer patients. We retrospectively reviewed 193 out of 215 advanced lung cancer patients who were consecutively treated with cytotoxic chemotherapy at Seoul National University Hospital in 2005. The mean age was 61 years (range, 31-85 years). A total of 41 patients (21%) had Eastern cooperative oncology group (ECOG) performance status 2-3. Among 135 patients with pulmonary function test (PFT), 34% (n=46) of patients demonstrated an obstructive pattern of lung function. Pneumonia occurred in a total of 10 patients (5%). Occurrence rate was significantly higher in older patients (>70 years) than in younger ones (13% vs. 4%, p=0.05), higher in patients with ECOG 2-3 compared with those with ECOG 0-1 (15% vs. 3%, p<0.01), and higher in patients with an obstructive pattern of lung function compared with those without (11% vs. 1%, p=0.02). Pneumonia occurrence rate in patients with all three risk factors was 25%. Six out of 10 pneumonia patients required hospitalization, and one patient died from respiratory failure. We identified this high-risk group as a candidate for antibacterial prophylaxis after cytotoxic chemotherapy in advanced lung cancer patients.