OBJECTIVE. This study retrospectively evaluates complications after lung radiofrequency ablation (RFA). MATERIALS AND METHODS. Complications were assessed for each RFA session in 420 consecutive patients with 1403 lung tumors who underwent 1000 RFA sessions with a cool-tip RFA system. A major complication was defined as a grade 3 or 4 adverse event. Risk factors affecting frequent major complications that occurred with an incidence of 1% or more were detected using multivariate analysis. RESULTS. Four deaths (0.4% [4/1000]) related to RFA procedures occurred. Three patients died of interstitial pneumonia. The other patient died of hemothorax. The major complication rate was 9.8% (98/1000). Frequent major complications were aseptic pleuritis (2.3% [23/1000]), pneumonia (1.8% [18/1000]), lung abscess (1.6% [16/1000]), bleeding requiring blood transfusion (1.6% [16/1000]), pneumothorax requiring pleural sclerosis (1.6% [16/1000]), followed by bronchopleural fistula (0.4% [4/1000]), brachial nerve injury (0.3% [3/1000]), tumor seeding (0.1% [1/1000]), and diaphragm injury (0.1% [1/1000]). Puncture number (p < 0.02) and previous systemic chemotherapy (p < 0.05) were significant risk factors for aseptic pleuritis. Previous external beam radiotherapy (p < 0.001) and age (p < 0.02) were significant risk factors for pneumonia, as were emphysema (p < 0.02) for lung abscess, and serum platelet count (p < 0.002) and tumor size (p < 0.02) for bleeding. Emphysema (p < 0.02) was a significant risk factor for pneumothorax requiring pleural sclerosis. CONCLUSION. Lung RFA is a relatively safe procedure, but it can be fatal. Risk factors found in this study will help to stratify high-risk patients.