Introduction: Women who smoke, deliver significantly smaller infants. These infants have reduced levels of the vasodilator endothelial nitric oxide synthase (eNOS) levels in the umbilical vessels, which may reduce fetal growth. Serum cotinine, the degradation product of nicotine, can be used to determine the level of tobacco exposure. Newborns of environmental smokers are suggested to be smaller and shorter in weight, length, and head circumference. eNOS levels have not yet been studied in these infants. We investigated the existence of a relation between maternal environmental tobacco smoke exposure, eNOS activity, concentration, and birthweight. Material and methods: We included 263 healthy singleton pregnancies categorized into three groups according to measured cotinine levels: 175 nonsmokers, 38 smokers, and 50 environmental smokers. Cotinine was quantified by mass spectrometry with a detection limit of.2 ng/mL; eNOS activity and concentration were measured in endothelial cells (ECs) of the umbilical vein. Results: Infants born to environmental smokers had similar weights to infants born to nonsmokers (47 g heavier, P =.48). Cotinine concentrations were.06/.09/.12 ng/mL (quartiles) in infants born to nonsmokers,.27/.37/.81 ng/mL in infants born to women exposed to environmental tobacco smoke, and 43.0/63.8/108.1 ng/mL in infants born to smokers. The eNOS concentration was 1.65 ±.92 ng/10 ⁶ ECs (mean ± SD) in nonsmokers and 1.71 ± 1.00 ng/10 ⁶ ECs in environmental smokers. The eNOS activity was 52.0 ± 20.6 pmol l-citrulline/min/10 ⁶ ECs in nonsmokers and 48.7 ± 19.8 pmol l-citrulline/min/10 ⁶ ECs in environmental smokers. Conclusions: Infants born to environmental smokers, as judged by umbilical serum cotinine levels close to.2 ng/mL, are not associated with lower birthweight or reduced eNOS activity, or concentration in the fetal vascular bed.