PURPOSE: Postdural puncture headache is one well-known complications of spinal anesthesia. The development of postdurals puncture headaches is related to needle size, the direction of the needle bevel, the number of dural punctures and the age of the patient. However, the effect of the sitting position after spinal anesthesia (saddle block) on the incidence of postdural puncture headaches is not yet known. This study was performed to compare the incidence rates of headaches between a saddle block group (sitting position group) and a low spinal anesthesia group (supine position group). In addition, this study was performed to identify the predisposing factors influencing postdural puncture headaches (age, sex, seasonal variation, onset of headache, location of headache). METHODS: The authors analyzed 960 anal-surgery patients who were operated on using a saddle block or low spinal anesthesia at Hang Cinic from Jan. 2000 through Dec. 2000. The authors compared the incidence rates of headaches between the saddle block group (480 cases) and the low spinal anesthesia group (480 cases). RESULTS: 1) The incidence of postdural puncture headaches was not significantly different between the two groups (2.5% in the saddle block group, and 2.3% in the low spinal anesthesia group) (P>0.05). 2) The postdural puncture headache incidence rate was higher for younger patients (20~30 years) and for females. (M:F=7:16) (P<0.05). 3) The onest of postdural puncture headaches was at the postoperative 2nd day in 16 cases (70%) and at the postoperative 3rd day in 6 cases (26%). 4) The incidence rate of postdural puncture headache was higher in the summer (5 cases in June, and 3 cases in July, 4 cases in August). 5) The headache were located in the frontal region in 16 cases (70%) and in the occipital region in 3 cases (13%). CONCLUSIONS: There is no significant difference in the incidence rates of postdural puncture headaches between the saddle block group and the low spinal anesthesia group. Postdural puncture headaches had a tendency to occur more frequently in young female patients and during the summer season. In addition, a more intensive study of the use of saddle block anesthesia for ambulatory anal surgery is required.