A randomized study was carried out to determine whether the administration of oxygen (3 liter/min) via nasal prongs significantly affects arterial oxygenation during colonoscopy in patients sedated with intravenous midazolam (2.6 +/- 0.2 mg, means +/- SE) and meperidine (48 +/- 3 mg). Patients who received supplemental oxygen were less likely to become hypoxic (pulse oximeter reading, SpO2 less than 90%) than those who breathed room air (10 of 28 vs. 22 of 28, p less than 0.005). Similarly, the total time during which SpO2 was below 90% was significantly less in patients receiving nasal oxygen (0.7 +/- 0.3 min) than in patients breathing room air (9.7 +/- 1.9 min, p less than 0.001). Minimum oxygen saturations were significantly higher in patients receiving oxygen (90.6 +/- 0.8%) than in patients breathing air (86.5 +/- 0.8%, p less than 0.001). In patients breathing air, there was a significant negative correlation between the dose of meperidine and the minimum observed oxygen saturation; conversely, midazolam dose did not correlate with indices of hypoxemia. The authors conclude that administration of oxygen via nasal prongs can reduce the risk of hypoxemia during colonoscopy. However, since hypoxemia may occur even when nasal oxygen is given, continuous monitoring of arterial oxygenation is recommended.