Our goal for this study was to determine whether the maturation of fat absorption in neonatal life is functionally related to an increased ability to hydrolyze dietary fat, to absorb long-chain fatty acids, or to do both. In 16 preterm and in eight term neonates, the intestinal ability to hydrolyze triacylglycerols and the capacity to absorb long-chain fatty acids were determined at several times between birth and 5 mo after the term age. These processes were compared with the percentage of fat absorption (formula-fed infants) or with fecal fat excretion (breast-fed infants). The functional capacity to digest triacylglycerols and to absorb the lipolytic products was evaluated by measuring serum concentrations of the lipolytic product [1-13 C]palmitate after the enteral administration of tri-1-13 C palmitoyl-glycerol. Longchain fatty acids absorption (i.e. independent of lipolysis) was determined by measuring serum concentrations of [1-13 C]stearate after its enteral administration. The efficacy of fat absorption increased in preterm infants (formula-fed) from 91.2 Ϯ 1.1% (mean Ϯ SEM) at 32. Efficient absorption of fat from the diet is important for energy supply, growth, and development, especially during development (1). At the neonatal age, and particularly in the case of prematurity, a considerable part of dietary fat is not absorbed from the intestine, but is excreted via the feces (2-5). Fat absorption involves digestion, requiring lipolysis by lipolytic enzymes, and the subsequent intestinal absorption of the hydrolyzed fatty acids and monoacylglycerols (6). The uptake of lipolytic products encompasses intraluminal solubilization of lipolytic products, their subsequent translocation across the small intestine, and their incorporation into chylomicrons that are subsequently secreted into the lymph (7). The importance and relative contribution of incomplete digestion and a decreased ability for fatty acid absorption to the causes of increased fecal fat loss during (preterm) infancy is only partly understood. Several studies suggest that lipolysis of fats could be the rate-limiting step in (preterm) neonatal dietary fat absorption. In the human gastrointestinal system several lipases are active on dietary triacylglycerols: gastric lipase (8, 9), pancreatic (colipase-dependent) lipase (10, 11), and bile salt-stimulated lipase (12). Several investigators have demonstrated a decreased intraduodenal concentration of both pan-