This study examined the in-vivo effect of the NSAID celecoxib on DNA methylation in the promoter region of the tumor-suppressor genes cadherin 13, tissue factor pathway inhibitor 12, and follistatin-like protein 1, and on apoptosis, in esophageal squamous cell carcinoma (ESCC). Forty-five patients who underwent an esophagectomy for ESCC were allocated to either a treatment group (n=22) or a control group (n=23). Patients in the treatment group were administered 800 mg/day of celecoxib for 14 days before surgery. Patients in the control group did not take any type of NSAID. Biopsies of the tumor were collected before surgery and tissue from the resection specimens after surgery. Methylation-specific PCR was used to measure DNA methylation and apoptosis was measured by flow cytometry. There was no difference in the proportion of patients with methylation for each of the genes between the patient groups before treatment. In those patients with pretreatment methylation, there was a significant reduction in the proportion with methylation and a significant increase in the corresponding messenger RNA expression after treatment with celecoxib. In those tissues in which there was a reduction in methylation following celecoxib treatment, there was a significant increase in the percentage of apoptotic cells, but not in the tissues with no change in methylation. In ESCC, in-vivo treatment with celecoxib is associated with a reduction in DNA methylation and increase in messenger RNA expression of tumor-suppressor genes, and increases in apoptosis.