The influence of the mode of delivery on the umbilical cord serum aldosterone levels in preterm newborn infants was studied. Sixty-four newborn infants gestational age less than 37 weeks were sequentially included in the study. Umbilical cord blood was collected just after birth and the aldosterone level was measured by radioimmunoassay. The median aldosterone level of the whole studied sample was 74.5 ng/dl (range: 22.00-280.00 ng/dl). The aldosterone level of the 31 newborn infants delivered vaginally was similar of the 33 newborn infants delivered by cesarean-section. When the cesarean group was subdivided in presence or not of labor prior to delivery, the aldosterone levels of those delivered after labor was significantly higher than those without labor. It is suggested that some event associated to the mode of delivery or the presence of labor prior to delivery may influence aldosterone levels in preterm newborn infants.