Diflumidone is a non-steroidal antiinflammatory drug. It inhibits the biosynthesis of prostaglandin by bovine seminal vesicle microsomes and arachidonic acid-induced aggregation of human platelets in a concentration-dependent manner. Diflumidone is a competitive inhibitor of prostaglandin synthetase. Diflumidone also inhibits equally the formation of PGE2 and PGF2a, which suggests blockade of endoperoxide formation. The relative topical efficacy of indomethacin and diflumidone for the suppression of ultraviolet-light-induced erythema has been compared in a randomized, double-blind, placebo-controlled study in man. At 24 h after application, the indomethacin-treated sites had significantly less erythema than did the diflumidone-treated sites.