Recent epidemiologic studies have shown a 40–50% reduction in mortality from colorectal cancer in individuals who take nonsteroidal antiinflammatory drugs on a regular basis compared with those not taking these agents. One property shared by all of these drugs is their ability to inhibit cyclooxygenase (COX), a key enzyme in the conversion of arachidonic acid to prostaglandins. Two isoforms of COX have been characterized, COX-1 and COX-2. COX-2 is expressed at high levels in intestinal tumors in humans and rodents. Human colon cancer cells (Caco-2) were permanently transfected with a COX-2 expression vector or the identical vector lacking the COX-2 insert. The Caco-2 cells, which constitutively expressed COX-2, acquired increased invasiveness compared with the parental Caco-2 cells or the vector transfected control cells. Biochemical changes associated with this phenotypic change included activation of metalloproteinase-2 and increased RNA levels for the membrane-type metalloproteinase. Increased invasiveness and prostaglandin production were reversed by treatment with sulindac sulfide, a known COX inhibitor. These studies demonstrate that constitutive expression of COX-2 can lead to phenotypic changes that alter the metastatic potential of colorectal cancer cells.