Data obtained with stable isotope methodology have demonstrated that preterm and term infants can convert LA and ALA, respectively, to AA and DHA. In addition, they have clarified the pathways by which infants convert LA and ALA to LCPUFA and have demonstrated the importance of factors such as the dietary LA/ALA ratio and postnatal age on biosynthesis of AA and DHA. Further work is needed to clarify the role of other influential factors on endogenous synthesis of LCPUFA and to determine the absolute amounts of endogenous LCPUFA synthesis. Such data are necessary to define more precisely the LCPUFA requirements of growing infants.