BACKGROUND: Controversy exists as to whether or not inhalation anesthetics and intravenous anesthetics impair arterial oxygenation (PaO2) during one lung ventilation (OLV). Accordingly, we examined the effect of enflurane and propofol on PaO2 and pulmonary vascular resistance (PVR) during OLV. METHODS: Forty patients, who had prolonged periods of OLV anesthesia with minimal trauma to the nonventilated lung were studied in a cross over design. Patients were randomized to four groups; Group 1 received 1 MAC of enflurane and oxygen from induction until the first 20 min after complete lung collapse, then were switched to propofol 100 g/kg/min (P100). In group 2, the order of the anesthetics was reversed. Group 3, Group 4 received the same order of the anesthetics as Group 1, Group 2, respectively but received propofol 200 g/kg/min (P200). RESULTS: During OLV, the PaO2 values were lower than those with two lung ventilation (TLV), there were no significant differences among each groups and between propofol and enflurane in PaO2, but in the selected patients (n=10, PaO2<120 mmHg during OLV), PaO2 in propofol group was higher than that of enflurane group (p<0.05). Conversion from TLV to OLV caused a significant increase in PVR, but there were no difference in PVR between propofol and enflurane group. CONCLUSIONS: These results suggest that the usual clinical dose of propofol affords no advantage over 1 MAC of enflurane anesthesia except low PaO2 patients during OLV. Propofol might be of value in risk patients of hypoxemia during thoracic surgery when OLV is planned.