Cyclooxygenase is a key enzyme responsible for metabolisation of arachidonic acid into prostaglandins and thromboxane. This enzyme is the target of non steroidal anti-inflammatory drugs (NSAIDs), used against inflammation and pain. The inducible COX-2 was associated with inflammatory conditions, whereas the constitutive form (COX-1) was responsible for the beneficial effects of the PGs. This observation led to the development of COX-2 inhibitors or “coxibs” of which rofecoxib (Vioxx®) characterized by a methylsulfone moiety and the sulfonamides celecoxib (Celebrex®) and valdecoxib (Bextra®). Initially described as COX-2 “selective” inhibitors, recent reports revealed a nanomolar inhibition activity of the sulfonamide COX-2 inhibitors for several carbonic anhydrase (CA) isoforms, confirmed by X-ray crystal structures for the adducts of celecoxib and valdecoxib with isozyme CA II. This dual activity may help to explain differences in clinical observation between sulfonamide and methylsulfone COX-2 inhibitors. Moreover, the inhibition of CA isozymes, critical for the development and invasion of cancer cells, such as CA II, IX and XII, may constitute an important mechanism of antitumor action of such sulfonamide compounds.