Pharmacological inhibition of cyclooxygenase-2 (COX-2) is being explored as a chemotherapeutic option because COX-2 protein expression is often elevated in many cancers. Cancer cells treated with COX-2 inhibitors, such as the selective COX-2 inhibitor celecoxib, show growth inhibition and the induction of apoptosis, through alterations in inflammatory processes, angiogenesis, cell adhesion and transforming growth factor- signaling. This study was conducted to determine if the same processes are relevant to celecoxibs effects on human colorectal adenocarcinomas treated in vivo. A cohort of 23 patients with primary colorectal adenocarcinomas was randomized to receive a 7-day course of celecoxib (400 mg b.i.d.) or no drug prior to surgical resection. Gene expression profiling was performed on resected adenocarcinomas from patients with and without celecoxib pre-treatment. Using fold change (>1.5) and p-value (<0.05) cut-offs, 190 genes were differentially expressed between adenocarcinomas from patients receiving celecoxib and those that did not. Of the differentially expressed genes, multiple genes involved in cellular lipid and glutathione metabolism showed decreased expression levels in celecoxib pre-treated samples; changes associated with diminished cellular proliferation. Other observed gene expression changes consistent with reduced proliferation include: altered expression of genes involved in cell adhesion (including collagen, laminin, von Willebrand factor and tenascin C), increased expression of inflammatory modulators (including inerleukin-6, S100 calcium binding protein A8, and several chemokines) and decreased expression of the pro-angiogenic gene, angiogenin. Celecoxib pre-treatment for 7 days in vivo is associated with alterations in colorectal adenocarcinoma gene expression which are suggestive of diminished cellular proliferation.