In our previous study, the proliferation rate of esophageal squamous cell carcinoma cell lines, which poorly expressed p21Waf1, was found to be regulated by p21Waf1 gene transfection using adenovirus vector. In the present study, in order to examine the effect of p21Waf1 gene therapy in esophageal cancer, we used gene gun technology, which proved to be a powerful method to introduce the p21Waf1 gene into esophageal cancer cells. p21Waf1 transfection to KE3 and YES2 cells (weakly expressed p21Waf1 protein cells) showed a high expression of p21Waf1 protein after applying this gene gun technique. In KE3 and YES2 cells, statistical significant growth inhibition was observed after p21Waf1 transfection compared with LacZ transfection (KE3, p=0.0009; YES2, p<0.0001). In in vivo transfection experiments, on day 14, the estimated volume of KE3 tumors subjected to p21Waf1 gene transfection was 95% in comparison with the pretreatment volume on day 0, while the volume of KE3 tumors subjected to LacZ gene therapy increased to 268%. On day 14, the estimated volume of YES2 tumors subjected to either p21Waf1 or LacZ gene therapy increased to 474 and 686%, respectively. In KE3 and YES2 cells, significant growth inhibition was observed after combination therapy using p21Waf1 transfection and anticancer drug 5-fluorouracil (5Fu) compared with 5Fu alone (KE3, p<0.0001; YES2, p<0.0001). In conclusion, p21Waf1 gene therapy using the gene gun technique significantly inhibited the low basal p21Waf1 expressed esophageal cancer cell growth in vitro and in vivo. Furthermore, p21Waf1 transfection strongly enhanced the effect of 5Fu suggesting that p21Waf1 may prove beneficial in chemotherapy combined with gene therapy using gene gun technology in patients with esophageal cancer who have a low level of p21Waf1 expressed tumor.