Abstract. Objective: Fetuses are exposed to high concentrations of estradiol due to placental production. Experimental datasuggest that estradiol is an important modulator of the immune response. However, the role of estradiol in the pathogenesis ofearly-onset neonatal sepsis (EOS) is unknown. The purpose of this pilot study was to determine estradiol levels in umbilicalvenous blood of newborns with EOS or chorioamnionitis exposure. Methods: Estradiol concentrations were measured by enzyme immunoassay in 37 newborns with EOS, 37 newborns with chorioamnionitis and 37 controls matched for gestational age and gender. Results: Estradiol levels correlated with gestational age, birth weight, gender and mode of delivery (p < 0.05). Multivariate analysis revealed higher estradiol levels in the EOS than in the chorioamnionitis group (odds ratio 8.43, 95% CI 1.63–43.45, p = 0.01) with the highest levels in patients with proven bacteraemia (p = 0.02). No difference was found between the EOS and the control group. Exploratory analysis showed an association between lower estradiol levels and a longer duration of mechanical ventilation (n = 28, p = 0.02). Conclusion: Umbilical venous estradiol levels were similar in EOS compared to controls. Further investigation is needed to evaluate whether high estradiol levels in infants with chorioamnionitis increases the risk of developing EOS.