BACKGROUND:Recent work has demonstrated a correlation between frequency of aspirin ingestion and colorectal cancer prevention. Sulindac, another nonsteroidal anti‐inflammatory drug (NSAID), has been shown to cause polyp regression and a fall in cell proliferation in patients with familial adenomatous polyposis, who are destined to develop colorectal cancer unless the colon is removed. However, the mode of action of NSAIDs in colorectal carcinogenesis prevention remains to be determined, although a prostaglandin‐mediated mechanism seems likely.METHODS:Rectal or duodenal biopsies from 20 patients with familial adenomatous polyposis, who had been randomized to sulindac or placebo, were analyzed for prostaglandin (PG) E2and E2&agr;levels before and after treatment.RESULTS:A significant fall in prostaglandin E2and E2&agr;(P=0.0096; PGE2, P=0.036; PGF2&agr;Spearman's rank correlation).CONCLUSIONS:Nonsteroidal antiinflammatory drugs may prevent colorectal cancer by their inhibition of prostaglandin synthesis. Prostaglandins may be implicated in carcinogenesis through an increase in cell proliferation, through immunosuppression, by increasing neovascularization, or via a mutagenic effect.