To retrospectively evaluate effectiveness, follow-up imaging features, and safety of microwave ablation in 50 patients with intraparenchymal pulmonary malignancies. This HIPAA-compliant study was approved by the institutional review board; informed consent was waived. From November 10, 2003, to August 28, 2006, 82 masses (mean, 1.42 per patient) in 50 patients (28 men, 22 women; mean age, 70 years) were percutaneously treated in 66 microwave ablation sessions. Each tumor was ablated with computed tomographic (CT) guidance. Follow-up contrast material-enhanced CT and positron emission tomographic (PET) scans were reviewed. Mixed linear modeling and logistic regression were performed. Time-event data were analyzed (Kaplan-Meier survival estimates and log-rank statistic). All event times were the time to a patient's first event (alpha level = .05, all analyses). At follow-up (mean, 10 months), 26% (13 of 50) of patients had residual disease at the ablation site, predicted by using index size of larger than 3 cm (P = .01). Another 22% (11 of 50) of patients had recurrent disease resulting in a 1-year local control rate of 67%, with mean time to first recurrence of 16.2 months. Kaplan-Meier analysis yielded an actuarial survival of 65% at 1 year, 55% at 2 years, and 45% at 3 years from ablation. Cancer-specific mortality yielded a 1-year survival of 83%, a 2-year survival of 73%, and a 3-year survival of 61%; these values were not significantly affected by index size of larger than 3 cm or 3 cm or smaller or presence of residual disease. Cavitation (43% 35 of 82 of treated tumors) was associated with reduced cancer-specific mortality (P = .02). Immediate complications included pneumothorax (Common Terminology Criteria for Adverse Events CTCAE grades 1 18 of 66 patients and 2 eight of 66 patients), hemoptysis (four of 66 patients), and skin burns (CTCAE grades 2 one of 66 patients and 3 one of 66 patients). Microwave ablation is effective and may be safely applied to lung tumors. (c) RSNA, 2008.