The indication for oxygen supplementation during diagnostic fibreoptic bronchoscopy (FOB) in local anaesthesia was evaluated by means of pulse oximetry in 160 patients (108 men, 52 women), median age 62 years. The patients were allocated at random into four groups of each 40 persons, which were comparable concerning pulmonary function and the dose of benzodiazepine used for premedication. The oxygen saturation of the haemoglobin (SpO2) in the tip of the index finger and the pulse rate were continuously recorded. Group 1 was examined without O2 supplement. Group 2a received O2 2 litres/min through a catheter placed in the vestibulum nasi. Group 2b received O2 2 litres/min through a pharyngeal catheter. Group 3 received O2 3 litres/min through a pharyngeal catheter. SpO2 mean values during FOB were 92 +/- (SD) 3% in group 1 and 96 +/- 2% in groups 2a, 2b and 3 (p < 0.001). SpO2 trough levels were mean 87 +/- 4% in group 1 vs. 93 +/- 2% in groups 2a and 2b, and 94 +/- 3% in group 3 (p < 0.001). Transient hypoxaemia, i.e., SpO2 < 85%, occurred with a frequency of 35% in group 1, 2.5% in group 2a, 0% in group 2b, and 2.5% in group 3. Tachycardia and bradycardia during FOB occurred with a frequency of 20% in group 1, 25% in group 2a, 18% in group 2b, and 10% in group 3. Pulse oximetry increases the safety of FOB, and is recommended for routine use. During FOB, oxygen supplement 2-3 litres/min should be administered to all patients, preferably through a pharyngeal catheter, as a preventive measure against hypoxaemia.