Although nonsteroidal antiinflammatory drugs (NSAIDs) show great promise as therapies for colon cancer, a dispute remains regarding their mechanism of action. NSAIDs are known to inhibit cyclooxygenase (COX) enzymes, which convert arachidonic acid (AA) to prostaglandins (PGs). Therefore, NSAIDs may suppress tumorigenesis by inhibiting PG synthesis. However, various experimental studies have suggested the possibility of PG-independent mechanisms. Notably, disruption of the mouse group IIA secretory phospholipase A₂ locus (Pla2g2a), a potential source of AA for COX-2, increases tumor number despite the fact that the mutation has been predicted to decrease PG production. Some authors have attempted to reconcile the results by suggesting that the level of the precursor (AA), not the products (PGs), is the critical factor. To clarify the role of AA in tumorigenesis, we have examined the effect of deleting the group IV cytosolic phospholipase A₂ (cPLA₂) locus (Pla2g4). We report that Apc^Min/+, cPLA₂^−/− mice show an 83% reduction in tumor number in the small intestine compared with littermates with genotypes Apc^Min/+, cPLA₂^+/− and Apc^Min/+, cPLA₂^+/+. This tumor phenotype parallels that of COX-2 knockout mice, suggesting that cPLA₂ is the predominant source of AA for COX-2 in the intestine. The protective effect of cPLA₂ deletion is thus most likely attributed to a decrease in the AA supply to COX-2 and a resultant decrease in PG synthesis. The tumorigenic effect of sPLA₂ mutations is likely to be through a completely different pathway.