The effectiveness of single prong nasal continuous positive airway pressure (CPAP) was compared with the Infant Flow Driver (IFD) in a crossover study in 20 neonates treated with > or = 30% oxygen by nasal CPAP. They were randomized to the device used at the start of the study. Each infant was studied for four consecutive 2-h periods alternating between single prong nasal CPAP and the IFD. The FiO2 from the IFD read 0.02 higher than the same setting on the ventilators used for single prong nasal CPAP. The IFD improved the mean (95% CI) of the FiO2 by 0.05 (0.02-0.08), p = 0.008. Taking into account the systematic error in the FiO2 between the devices the real mean improvement in FiO2 produced by the IFD was 0.03 (-0.005 to 0.06), p=0.09. There were no significant differences in respiratory rate, heart rate, blood pressure or comfort score of infants during periods of single nasal prong CPAP compared with periods on the IFD.