BACKGROUND: Anorectal procedures are performed in the jack-knife position. The combined use of midazolam and spinal anesthesia is common in clinical practice. Despite the known potential for each to alter ventilation, the effect of their interaction under jack-knife position has not been examined. METHODS: In a prospective, double-blind, and randomized study, forty patients (four groups, n=10 each, saline-supine position group, saline-jack-knife position group, midazolam-supine position group and midazolam-jack-knife position group) were examined. The effect of intravenous midazolam (0.05 mg/kg) for sedation, spinal anesthesia (hyperbaric tetracaine, below T8), position (jack-knife position or supine position during operation), and their combination on mean arterial pressure, arterial oxygen saturation (SpO2), PaO2, PaCO2, respiratory rate were evaluated. Incidence of hypoxemia was measured by pulse oximetry (SpO2 less than 90% for 30 seconds or longer). RESULTS: The incidence of hypoxemia in the midazolam-supine position group was 20%. There was no patient showing hypoxemia in other groups. After spinal anesthesia, there were no significant differences of mean arterial pressure, arterial oxygen saturation, PaO2, PaCO2, respiratory rate between supine and jack-knife position. The combination of jack-knife position and midazolam caused a significant increase of PaCO2 (9% of baseline value). Cardiovascular side effects such as hypotension or other reactions such as vomiting, nausea, or confusion were not observed after midazolam. At the end of the operation, all patients were fully awake and cooperative. CONCLUSIONS: Intravenous midazolam (0.05 mg/kg) may produce hypoxemia (SpO2<90%) during spinal anesthesia in supine position. In the jack-knife position, intravenous midazolam caused increase of the arterial CO2 tension. Monitoring of arterial blood oxygen saturation is mandatory in patients with spinal anesthesia and midazolam sedation during supine position.