The time taken for the oxygen saturation (SpO2) to decrease to 90% after preoxygenation was studied in six morbidly obese patients and six matched controls of normal weight. During apnoea the obese patients maintained Spo2 greater than 90% for 196 (SD 80) s (range 55-208 s), compared with 595 (SD 142) s (range 430-825 s) in the control group (P less than 0.001). One patient in the obese group had desaturation before the onset of complete relaxation and tracheal intubation, without complications. Bedside lung function tests were not significantly different between groups and cannot be used as a predictor of the effectiveness of preoxygenation.