Use of nonsteroidal anti-inflammatory drugs such as aspirin, which are known to inhibit cyclooxygenase activity, reduces the relative risk of colorectal cancer in humans by 40-50%. Animal and human studies have shown a 50-80% reduction in tumour multiplicity following treatment with a variety of nonsteroidal anti-inflammatory drugs. Two isoforms of cyclooxygenase have been described, cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2). In 85% of colorectal adenocarcinomas taken from humans. COX-2 levels are 2-50-fold higher than levels in adjacent normal intestinal mucosa, while COX-1 levels are unchanged. These observations raise the question: Does COX-1 or COX-2 provide a useful target for prevention or treatment of colorectal cancer?