We have investigated whether the incidence of neonatal pneumothorax (NP), use of mechanical ventilation (MV) or Continuous Positive Airways Pressure (CPAP) is increased in neonates delivered at term and preterm by cesarean section (CS). All deliveries at Ullevål University Hospital, Oslo, during the period 2001-2005 (n=29,358) were included, among whom 5,957 were delivered by CS (20.3%). Data were collected on mode of delivery, elective or emergency CS, gestational age, maternal age, gender and parity. Among the 26,664 neonates born at term (> or =37th gestational week), 4,546 were delivered by CS (17.0%), of whom 0.5% by elective and 0.6% by emergency CS with NP. The incidence of diagnosed NP was significantly higher after CS than after vaginal delivery (0.6% vs. 0.10%, p<0.001). In addition, the need for MV was significantly increased (0.41% vs. 0.19%; p=0.01) but use of CPAP was not (0.28%vs. 0.15%; p=0.08). Among 2,694 neonates born preterm (<37th gestational week), 1,266 were delivered by CS (47.0%). The incidence of diagnosed NP was 2.05% when delivered by CS but only 0.63% when delivered vaginally (p<0.01). Among the preterm infants delivered by CS, 17.7% needed CPAP compared to 6.9% when delivered vaginally (p<0.001) and MV was required for 8.1 and 3.7% (p<0.001), respectively. Among neonates delivered at term or moderately preterm (30-36 weeks) by CS the incidence of NP and other respiratory problems was significantly increased.