Nearly 30 years ago, cyclooxygenase (COX) was identified as an enzyme that initiates the biotransformation of arachidonic acid to prostanoids. It is now known that COX exists as two distinct but similar isozymes, COX-l and COX-2. Prostaglandins (PGs) formed by the enzymatic activity of COX-l are primarily involved in the regulation of homeostatic functions throughout the body, whereas PGs formed by COX-2 primarily mediate pain and inflammation. Based on structural differences in the active sites of COX-l and COX-2, a new class of drugs has been developed that specifically inhibits COX-2 but not COX-l activity. By preserving the synthesis of homeostatic PGs, these specific inhibitors of COX-2 provide the clinical benefits of nonsteroidal anti-inflammatory drugs and minimize the consequences of nonspecific inhibition of PG synthesis.