This study was executed to measure the biosynthesis of arachidonic acid metabolic products in chronic periapical lesions, to compare the products among periapical granuloma, periapical cyst and chronic periapical abscess, and to understand the pathogensis of chronic periapical lesions. Tissues from 33 chronic periapical lesions of human teeth were enucleated during endodontic surgery. large part of each tissue was contained in liquid nitrogen immediately and the other was examined histologically. In histologically diagnosed 8 cases of periapical granuloma, 9 cases of periapical cyst and 8 cases of chronic periapical abscess. the tissues were homogenatecl and incubated with $_{14}C$-arachidonic acid. Lipid solvent extracts were separated by thin layer chromatography to be analyzed by autoradiography and TLC analyzer. 1. $TXB_2$, 6-keto-$PGF_1{alpha}$ and $PGE_2$, $LTB_4$, HETEs, and unidentified product which are metabolic products of arachidonic acid were measured in the tissues of chronic peripaical lesions. 2. In all of periapical granuloma, cyst and abscess, the conversion rate of HETEs among all products was the highest(P<0.05), and the percentage of HETEs in total converted products was also the highest(P<0.05). 3. The concentration of each arachidonic acid product was higher in chronic periapical absecss than in periapical granuloma and cyst(P<0.05). The concentration of $TXB_2$ and HETEs in periapical cyst were hight than in periapical granuloma. 4. The relative amounts of total products from lipoxygenase pathway to those from cyclo-oxygenase pathway were about 7 fold in chronic periapical lesions. There was no difference among periapical granuloma, cyst and abscess(P<0.05). The total amount of products from each pathway were higher in chronic periapical abscess than in periapical cyst and granuloma.