PURPOSE: The purpose of this study was to retrospectively determine the incidence of early enlarging cavitation after percutaneous radiofrequency ablation (RFA) of lung tumor and identify risk factors associated with their occurrence. PATIENTS AND METHODS: A total of 245 patients (140 men, 105 women; mean age, 62.7 ± 11.8 [SD] years; age range: 31-87 years) with 605 lung tumors who were treated using 401 RFA sessions from April 2010 to March 2020 were included. Of which, 31 patients with 38 early enlarging cavitation and control group (151 patients with 228 tumors) were analyzed. Early enlarging cavitation was defined as cavities with an enlarged size (>3 cm) occurring on ablated lesions within seven days after RFA. Incidence of cavitation, risk and post-procedural factors of occurrence, major adverse events (AEs) that occurred in RFA sessions with cavitation, and course of cavitation were evaluated. AEs were classified using the CIRSE classification system for complications. RESULTS: Thirty-eight cavities (6.3%, 38/605 tumors) in 31 patients occurred in an average of 3.1 ± 1.7 days (range, 1-7 days) after 32 RFA sessions. Distance from pleura ≥ 20 mm, contact with vessel ≥ 3 mm, multitined expandable electrode ≥ 3 cm, and bronchus ≥ 2 mm encompassed in the ablation zone were independent risk factors of occurrence. Fever ≥ 38.5 °C, white blood cell count ≥ 10,000/μl one day after RFA, and steroid therapy were independent post-procedural factors of occurrence. Twenty-four Grade 3 and two Grade 6 AEs occurred. Twenty-nine cavities disappeared within a mean duration of 111.9 ± 64.9 (SD) days (range: 44-274 days) and four remained with a mean follow-up of 279.2 ± 174 (SD) days; five patients were lost to follow-up. CONCLUSIONS: Early enlarging cavitation occurs in 6.3% of lung tumors treated with RFA and are associated with 26 major AEs in 32 sessions. Aforementioned procedural factors and post-procedural inflammation were significant risk factors of occurrence.