The aim of this study was to determine the degree and duration of hypoxaemia during bronchoalveolar lavage (BAL) and to examine the effect of supplemental oxygen on this response. Transcutaneous oxygen tension (PO2) was recorded continuously in 22 patients having bronchoscopy alone (Group 1), and during BAL in patients with a variety of connective tissue disorders. Thirty eight of these patients were breathing room air (Group 2) and 28 were given supplemental oxygen (Group 3). The mean fall in PO2 in Group 1 was 12 +/- 3 mmHg and the PO2 in these subjects rose promptly to the initial value once the bronchoscopy was finished. The mean falls in Groups 2 and 3 were 24 +/- 4 and 32 +/- 5 mmHg and the mean times taken for the PO2 values to return to baseline after the procedure were 47 +/- 9 and 53 +/- 10 minutes respectively. The PO2 fell to less than 60 mmHg in 76% of the patients in Group 2 but in only 25% of those in Group 3. It is recommended that supplemental oxygen be given to all patients having BAL during and for one hour after the procedure and that oxygenation be monitored continuously throughout the bronchoscopy. Arterial blood gases should be always measured prior to bronchoscopy and BAL. In those cases where it is not possible to maintain the PaO2 at 70 mmHg or more the procedure should be undertaken with great care and with due consideration of the risk of the hypoxaemia that may occur.