Summary 1.The fatty acid compositions of theserum chylomicrons and of adipose tissues of 15 children with cystic fibrosis of the pancreas and of 12 controls of similar ages but without clinical evidence of impaired intestinal digestion and absorption were determined. 2.The serum chylomicrons of childrenwith cystic fibrosis were found to contain increased amounts of palmitoleic and oleic acids and decreased amounts of palmitic, stearic, and linoleic acids. 3.Two types of fatty acid patterns of depot fat are described in children with cystic fibrosis: (a) In the case of little or no absorption of fat the composition of the depot fat reflects active endogenous lipogenesis from carbohydrate; this pattern is also observed in very young infants without cystic fibrosis. (b) In most of the older children with cystic fibrosis the impairment in fat absorption is rarely extreme. In them, the increased tendency to selective absorption of oleic acid is superimposed upon an increased endogenous lipogenesis to produce a pattern characterized by relatively high content of palmitoleic and oleic acids and relatively low content of palmitic, stearic, and linoleic acids. This pattern of depot fat is also observed in adults with the malabsorption syndrome. 4.Improvement in the absorption of fatin children with cystic fibrosis following the administration of Cotazym failed to produce a significant change in the abnormal pattern of the depot fat in the majority of the children studied for periods of 3 to 9 months. In fact, the content of oleic acid may continue to increase with Cotazym therapy. 5.In view of the demonstration that oleic acid and perhaps other monounsaturated fatty acids are more readily absorbed than most of the other fatty acids commonly found in foods, a diet with relatively high monounsaturated fatty acid content and low in saturated acids may be helpful in improving the nutritional status of these patients. 6.This study may also serve as a basisfor studying the relationship of diet, and dietary fat in particular, to coronary artery disease. Summary 1.The fatty acid compositions of theserum chylomicrons and of adipose tissues of 15 children with cystic fibrosis of the pancreas and of 12 controls of similar ages but without clinical evidence of impaired intestinal digestion and absorption were determined. 2.The serum chylomicrons of childrenwith cystic fibrosis were found to contain increased amounts of palmitoleic and oleic acids and decreased amounts of palmitic, stearic, and linoleic acids. 3.Two types of fatty acid patterns of depot fat are described in children with cystic fibrosis: (a) In the case of little or no absorption of fat the composition of the depot fat reflects active endogenous lipogenesis from carbohydrate; this pattern is also observed in very young infants without cystic fibrosis. (b) In most of the older children with cystic fibrosis the impairment in fat absorption is rarely extreme. In them, the increased tendency to selective absorption of oleic acid is superimposed upon an increased endogenous lipogenesis to produce a pattern characterized by relatively high content of palmitoleic and oleic acids and relatively low content of palmitic, stearic, and linoleic acids. This pattern of depot fat is also observed in adults with the malabsorption syndrome. 4.Improvement in the absorption of fatin children with cystic fibrosis following the administration of Cotazym failed to produce a significant change in the abnormal pattern of the depot fat in the majority of the children studied for periods of 3 to 9 months. In fact, the content of oleic acid may continue to increase with Cotazym therapy. 5.In view of the demonstration that oleic acid and perhaps other monounsaturated fatty acids are more readily absorbed than most of the other fatty acids commonly found in foods, a diet with relatively high monounsaturated fatty acid content and low in saturated acids may be helpful in improving the nutritional status of these patients. 6.This study may also serve as a basisfor studying the relationship of diet, and dietary fat in particular, to coronary artery disease.