Arterial oxygen saturation (SaO2) was monitored continuously during and immediately after sputum induction in 41 HIV positive patients with respiratory symptoms and in 20 symptomless medical and nursing staff, who acted as control subjects. Arterial oxygen desaturation (defined as SaO2 ≤ 92%) occurred during sputum induction and persisted for up to 20 minutes after the end of the procedure in 11 of the 20 patients with Pneumocystis carinii pneumonia and in nine of the 21 patients with other respiratory diagnoses. None of the control subjects showed oxygen desaturation. Neither the severity of chest radiographic abnormalities, the alveolar-arterial oxygen gradient (both measured before sputum induction), nor baseline SaO2 prospectively identified the patients who developed oxygen desaturation. Two patients, one with pneumocystis pneumonia, developed dyspnoea and had a fall in arterial oxygen saturation to 84% within 10 minutes of starting sputum induction. The procedure was abandoned in both patients and in two further patients, who developed severe nausea and retching but no oxygen desaturation. Sputum induction in HIV positive patients with respiratory symptoms may induce a fall in SaO2 that persists after this procedure. This may be important if other procedures are performed soon after sputum induction.