The fatty acid compositions of serum phospholipids and cholesterol esters and direct bilirubinemia were determined in 11 children with cholestasis due to extrahepatic biliary atresia. The levels of the different fatty acids in these lipid classes were compared with those of 22 appropriate controls and correlations with conjugated bilirubinemia were calculated. Significant differences were found in the levels of several fatty acids in these lipid classes, some of which were related to conjugated bilirubinemia. Relationships between fatty acids in phospholipids and cholesterol esters which exist in the control group were either absent or different in the patient group. The results found are compatible with the concept that malabsorption, overflow in blood of phospholipids, which are excreted in bile in healthy individuals, and liver disease per se contribute to the deviating fatty acid compositions. They suggest that administration in the diet may be required of preformed long chain polyunsaturated fatty acids in an easily absorbable form.