BACKGROUND: Propofol is used worldwide for its sedative effective; nonetheless, has the serious side effect of respiratory depression. An increased blood concentration of propofol is well known to be associated with increased respiratory depression. However, there are no studies of the effect site concentration inducing respiratory depression. The purpose of this study was to determine the effect site concentration inducing respiratory depression of propofol when sedating a patient after spinal anesthesia. METHODS: This study included thirty seven males who received operations with spinal anesthesia, which was performed on L3-4 and L4-5. All patients were monitored with the bispectral index and were continuously infused with propofol using target controlled infusion. Respiratory depression was diagnosed when one of the following was evident without upper respiratory obstructive signs: a greater than 20% increase of end tidal carbon dioxide from baseline pressure or pulse oximetry oxygen saturation lower than 95%. We obtained the EC5, EC10, and EC50 of the effect site propofol for respiratory depression. RESULTS: The EC5 of propofol for respiratory depression was 3.09 mcg/ml (95% CI, 2.60-3.58). The EC10 of propofol for respiratory depression was 3.18 mcg/ml (95% CI, 2.57-3.80). The EC50 of propofol for respiratory depression was 3.99 mcg/ml (95% CI, 2.36-5.61). CONCLUSIONS: The EC5, EC10, and EC50 of effect site propofol for respiratory depression during spinal anesthesia were 3.09 mcg/ml ,3.18 mcg/ml, and 3.99 mcg/ml, respectively.