We studied the influence of EIS on the respiro-circulatory condition of patients with portal hypertension. Subjects were fifty patients with portal hypertension who were successfully injected more than 5.0 ml of sclerotant into varices. A prospective randomized controlled trial was proposed to elucidate the effect of prophylactic administration of pure oxygen. Twenty-five patients inhaled pure oxygen (O2), remaining twenty-five patients did not during EIS. Respiro-circulatory function of patients was evaluated by monitoring O2 saturation, pulse rate and blood pressure during EIS. PaO2 was measured before and after EIS in seven patients without O2. EIS by the 5% ethanolamine oleate with iopamidol (EOI) was performed under X-ray monitoring. O2 saturation in patients without O2 inhalation was lowered, whereas that in patients with O2 inhalation was stable during EIS. O2 saturation during injection of EOI and after EIS in patients without O2 inhalation was significantly lower than that in patients with O2 inhalation. Pulse rate was significantly lower and a rise in blood pressure was smaller in patients given O2. No significant differences of PaO2 was noted before and after EIS. In conclusion, the monitoring of O2 saturation, pulse rate and blood pressure is necessary during EIS. Prophylactic administration of pure oxygen may be useful for prevention of hypoxic state and respiro-circulatory stabilization during EIS.