OBJECTIVE. Optimal nutrition is of utmost importance for the preterm infant's later health and developmental outcome. Amino acid requirements for preterm infants differ from those for term and older infants, because growth rates differ. Some nonessential amino acids, however, cannot be sufficiently synthesized endogenously. Cyst(e)ine is supposed to be such a conditionally essential amino acid in preterm infants. The objective of this study was to determine, at 32 and 35 weeks' postmenstrual age, Cyst(e)ine requirements in fully enterally fed very low birth weight preterm infants with gestational ages of <29 weeks. METHODS. Infants were randomly assigned to 1 of the 5 graded cystine test diets that contained generous amounts of methionine. Cyst(e)ine requirement was determined with the indicator amino acid oxidation technique ([1-C-13] phenylalanine) after 24-hour adaptation. RESULTS. Fractional [1-C-13] phenylalanine oxidation was established in 47 very low birth weight preterm infants (mean gestational age: 28 weeks +/- 1 week SD; birth weight: 1.07 kg +/- 0.21 kg SD). Increase in dietary Cyst(e)ine intake did not result in a decrease in fractional [1-C-13] phenylalanine oxidation. CONCLUSIONS. These data do not support the hypothesis that endogenous Cyst(e)ine synthesis is limited in very low birth weight preterm infants with gestational ages of <29 weeks, both at 32 and 35 weeks postmenstrual age. It is safe to conclude that Cyst(e)ine requirement is <18 mg/kg per day in enterally fed very low birth weight preterm infants who are older than 32 weeks' postmenstrual age and whose methionine intake is adequate. Therefore, Cyst(e)ine is probably not a conditionally essential amino acid in these infants.