Rapid aggressive tumor progression of hepatocellular carcinoma is one of the major complications after radiofrequency ablation. We statistically analyzed hepatocellular carcinoma patients with rapid aggressive tumor progression after radiofrequency ablation regarding patient characteristics, computed tomography findings, radiofrequency ablation methods, and the course of clinical and imaging changes and outcomes.We defined the tumor not detected by previous CT and US rapidly increased more than double in compared with the ablated area as rapid aggressive tumor progression. Ten hundred seventy three lesions of 538 patients underwent ultrasound-guided radiofrequency ablation in our hospital between April 1999 and March 2008. Of these patients, the risk factors for rapid aggressive tumor progression of 7 with 7 lesions were analyzed.The rate of rapid aggressive tumor progression was 0.65%. Subcapsular lesions, a pretreatment Protein Induced by Vitamin K Absence-II level > or = 40mAU/ml, and initial treatment for hepatocellular carcinoma were significantly identified as risk factors.There is possibility that portal vein invasion by hepatocellular carcinoma is an important factor involved in rapid aggressive tumor progression after radiofrequency ablation from the results of this study. In our opinion, these hepatocellular carcinomas need to be sufficiently ablated.