To avoid the possible confounding effects of postnatal exposure to tobacco smoke, we investigated possible effects of uterine tobacco smoke (UTS) exposure upon infant lung function shortly after birth. Infants with no major disease, in one maternity ward in Oslo, Norway, participating in a cohort study established in 1992/1993, were included in the present study (n=803). Exposure information, assessed as maternal active and passive smoking during pregnancy and other personal and environmental factors, was obtained by questionnaire. Tidal flow-volume (TFV) loops (n=802) and compliance (Crs) and resistance (Rrs) of the respiratory system (n=663) were measured at a mean age of 2.7 days. In girls, the TFV ratio (time to reach peak expiratory flow to total expiratory time (tPEF/tE)), and Crs were significantly lower with active as well as passive maternal smoking compared to nonexposure to UTS. Respiratory rate and Rrs were not significantly influenced by UTS exposure. However, in linear regression analysis adjusted for confounding factors (including respiratory rate), tPEF/tE and Crs, but not Rrs, were related to maternal active but not passive daily smoking. One daily cigarette corresponded to a change in tPEF/tE of -0.0021 (95% confidence interval (95% CI) -0.0040 to -0.0002) and a change in Crs of -0.026 mL x cmH2O (95% CI -0.045 to -0.007 mL x cmH2O). The decrease was 0.023 and 0.29, respectively, in infants of an average smoker. Maternal smoking during pregnancy adversely affected tidal flow-volume ratios in healthy newborn babies, as well as the compliance of the respiratory system in girls, independently of the reduced body size also resulting from maternal smoking.