OBJECTIVES--To establish the incidence and severity of arterial oxygen desaturation during transoesophageal echocardiography performed under light intravenous sedation; to determine which patients are at greatest risk; and to assess the effects of supplementary oxygen treatment. DESIGN--Prospective study of 150 patients referred for transoesophageal echocardiography. SETTING--Echocardiography laboratory in a tertiary cardiothoracic referral centre. MAIN OUTCOME MEASURE--Transcutaneous arterial oxygen saturation. RESULTS--During transoesophageal echocardiography mean (SD) arterial oxygen saturation (SaO2) fell in 144 of 150 patients (96%) from 95.4%(2.6%) to 90.7%(6.3%) (p < 0.001). Significant hypoxaemia, defined as SaO2 < 90%, was found in 27 of 150 patients (18%); in this group SaO2 fell from 92.9%(3.5%) to 81.8%(9.6%) (p < 0.001), but rose rapidly on oxygen to 95.5%(2.4%) (p < 0.001). Two patients became profoundly hypoxaemic with SaO2 values of 35% and 74%. The principal risk factors for hypoxaemia during transoesophageal echocardiography were mitral valve disease, severe mitral regurgitation, and New York Heart Association symptomatic class III or IV. CONCLUSIONS--Transcutaneous oximetry and supplementary oxygen should be available routinely during transoesophageal echocardiography.