The fatty acid composition of biliary phosphatidylcholine was analyzed in 13 patients with radiolucent gallstones undergoing elective cholecystectomy, and in 11 normolipemic patients without gallstones undergoing abdominal surgery. The only difference in the percentage fatty acid composition between the two groups was a significantly (p<0.05) higher percentage arachidonic acid in the first group. This acid was exclusively located in thesn-2 position of phosphatidylcholine (PC), accounting for 13.0±4.9% in the first group and 8.2±4.9% in the second (p<0.05). The percentage arachidonic acid of PC was negatively correlated (p<0.001) with the percentage biliary chenodeoxycholate in gallstone patients, but not in controls. Explanation of these findings is, at present, only speculative.