We measured preprandial and 2-h postprandial plasma triglycerides (TG) and free fatty acids (FFA) in 13 preterm infants weekly for 3 consecutive weeks to examine the effects of a premature, medium-chain triglyceride (MCT)-containing formula advancing to a term, long-chain triglyceride (LCT)-containing formula. The infants were on premature formula for the first 2 weeks of the study and were switched to term formula 4-5 days before the last sampling. Significant increases were seen in preprandial and 2-h postprandial TG when the infants were advanced from the premature to the term formula. It was speculated that this increase was due to the predominance of LCT in term formula, which differed in its absorption and metabolism from MCT. Whether these increases persist and if their clinical significance requires further investigation. No consistent change was observed in the preprandial or postprandial FFA.