Objective: The purpose of this study was to quantify differences in indexes of pulmonary maturity between singleton and twin gestations by means of the TDx fetal lung maturity assay. Study Design: We identified records of a total of 830 singleton and twin pregnancies not complicated by diabetes and delivered between 28 and 37 weeks’ gestation from December 1994 through August 1995. Among these, 170 (20%) had TDx fetal lung maturity measurements performed within 72 hours of delivery. Linear regression was used to assess differences in TDx fetal lung maturity assay values between singleton gestations (n = 143 gestations) and twin gestations (n = 27 gestations) while controlling for potential confounding factors. Results: Twin gestations were no more likely than singleton gestations to undergo TDx fetal lung maturity screening (odds ratio, 1.3; 95% confidence interval, 0.8-2.2). Pregnancy complications and corticosteroid treatment were similar in the two groups. After 31 weeks’ gestation the twin gestations had significantly higher TDx fetal lung maturity values. Linear regression with controls for gestational age indicated that twin gestations on average had a TDx fetal lung maturity value that was 22.0 mg/g (95% confidence interval, 9.8-34.6 mg/g) higher than that of gestational age–matched singleton gestations. Conclusion: Beyond 31 weeks’ gestation twin pregnancies appeared to have a TDx fetal lung maturity value that was 22 mg/g higher than that of singleton pregnancies. If the underlying incidences of respiratory distress syndrome are similar between twin and singleton gestations, then the potential exists for false-positive prediction of adequate lung maturity values among twin gestations. (Am J Obstet Gynecol 2000;182:1110-2.)