Serial arterial blood gases were measured during fibreoptic bronchoscopy in 26 patients with diffuse interstitial lung disease. All those having transbronchial biopsy with or without bronchoalveolar lavage, performed breathing room air, showed significant falls in PaO2, at the time of passage of the bronchoscope through the vocal cords both at the beginning and end of the procedure. Patients having bronchoalveolar lavage and transbronchial biopsy performed with supplementary oxygen at least maintained their basal PaO2 levels. Cessation of the supplementary oxygen 30 minutes after withdrawal of the bronchoscope did not result in rebound hypoxaemia. Bronchoalveolar lavage and transbronchial biopsy can be performed safely in patients with diffuse interstitial lung disease without significant hypoxaemia developing, if oxygen is started before the procedure and continued for 30 minutes after withdrawal of the bronchoscope.