Morbid obesity is associated with a reduction in time to desaturate during apnoea following standard pre-oxygenation and induction of anaesthesia. We have compared the effects of using 7.5 cmH2O of continuous positive airway pressure (CPAP) for pre-oxygenation with a standard technique using a Mapleson A breathing system, in 20 morbidly obese women. In a prospective, open, randomised trial, we measured the time taken to desaturate to 90% from time of giving a succinylcholine bolus as part of a rapid induction of anaesthesia. All patients received 3 min pre-oxygenation prior to induction. Tracheal intubation was confirmed and all patients kept apnoeic until oxygen saturation decreased to 90%. No statistically significant difference in mean time to desaturate to 90% could be demonstrated in the CPAP group compared to the Mapleson A group (240 s and 203 s, respectively). A brief period of lower mean heart rate in the CPAP group was the only statistically significant difference in cardiovascular parameters. There was no significant difference in the volume of gastric gas after induction between groups.