Since tumor cells are known to express heat shock proteins (HSPs) as a response to cellular stress, such as heat, our goal was to determine the expression of HSPs in human hepatocellular carcinoma (HCC) before and after percutaneous radiofrequency (RF) ablation using a rat model. In 12 nude rats, human HCC cells (SK-HEP-1) were inoculated subcutaneously. A total of 21 tumors were grown in the bilateral flanks of the rats. Of those, 19 were treated with percutaneous RF ablation (diameter of RF electrode, 18 gauge; RF ablation energy, 60-600 W; duration, 20-100 sec). To determine the extent of necrosis, and the cellular expression of HSP 70 and HSP 90, the tumors were excised within 6, 12 and 24 h after RF ablation, respectively. The extent of the coagulation necrosis and the expression of HSP 70 and 90 were evaluated. Linear regression analysis showed a significant correlation between the volume of coagulation necrosis and the RF energy applied. Before RF ablation, expression of HSP 70 and 90 was 0% and 0-30%, respectively. Following RF ablation, the maximum level of HSP 70 expression was 60%, and the maximum level of HSP 90 expression was 100%. The expression of HSP 70 and 90 in HCC is significantly increased by RF ablation. These findings are of particular importance in the host-tumor immune response and might be useful in forthcoming immunotherapeutical strategies.