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Introduction Use of the lowest effective dose of oxytocin may reduce side effects. This study was designed to determine the effective dose (ED) 90 of oxytocin infusion for an elective Cesarean delivery (CD) to prevent uterine atony. Methods The participants were ASA I and II, non-obese, non-labouring adult women undergoing an elective CD at term with a singleton gestation. The spinal anesthetic technique was standardized, and a blinded infusion of oxytocin was administered after delivery. The obstetrician rated the uterine contraction as either satisfactory or unsatisfactory. The initial dose of oxytocin infusion was 0.4 IUÁmin -1 , and the dose for the next subject was based on the response of the preceding subject as per a biasedcoin design up-down sequential method. The ED 90 was calculated using Firth's penalized likelihood estimation. Results Fifty subjects were screened, eight subjects were excluded, and two patients were withdrawn. Seven of the 40 subjects had uterine tone that was judged unsatisfactory by the obstetrician and required additional uterotonic medications. The ED 90 , i.e., the dose at which 90% of women were judged to have satisfactory uterine tone, was 0.29 IUÁmin -1 (95% confidence interval [CI] 0.15-0.43 IUÁmin -1 ). Discussion In this study, we found the ED 90 of oxytocin required to prevent uterine atony and postpartum hemorrhage after an elective CD to be 0.29 IUÁmin -1 -approximately 15 IU of oxytocin in 1 L of intravenous fluid administered over a one-hour period-(95% CI 0.15-0.43 IUÁmin -1 ). This oxytocin infusion dose is 30% less than the clinical infusions currently in use. It remains to be seen whether this dosing will be required for higher risk individuals or for labouring parturients undergoing nonelective CD. (Clinical Trial gov. NCT00785395). RésuméIntroduction L'utilisation de la dose efficace d'oxytocine la plus basse possible pourrait re´duire les effets secondaires. Cette e´tude a e´te´conçue pour de´terminer la dose efficace (DE) 90 d'une perfusion d'oxytocine afin de pre´venir l'atonie ute´rine apre`s un accouchement par ce´sarienne (AC) non urgent. Méthode Les participantes e´taient des femmes adultes ASA I et II, non obe`ses et pas encore en travail obste´trical subissant un AC a`terme d'une grossesse unique. La technique d'anesthe´sie rachidienne e´tait standardise´e,