PURPOSE: Urinary retention is a frequent postoperative complication after benign anorectal surgery. Factors, known to affect postoperative urinary retention, are age, sex, anesthetics, operative method, operative time and perioperative fluid injection. This study was performed to know whether the incidence of urinary retention might be controlled by reducing the amount of perioperative fluid. METHODS: Eighty patients underwent surgery for hemorrhoids and chronic anal fissures were allocated into two groups, fluid restriction group (n=37) and hydration group (n=43). All patients were consecutively randomized from May 1998 to January 1999 and they were under 50 years old without urologic abnormality. Fluid was infused at 100 ml/h from the midnight then it's rate was changed into 10 ml/h for 4 hours from the beginning of the anesthesia for the restriction group, whereas 1000 ml/h only during operation for the hydration group. Thereafter it was changed into the same rate with 100 ml/h on both groups. RESULTS: There was no significant differences with regard to age, sex, operation time, degree of pain and use of analgesics between two groups. Although there was a significant difference in the total volume of the infused fluid (Restriction group: 53.4 119.5 ml versus Hydration group: 778.6 319.0 ml, mean SD, p<0.001). Catheterization was done in 29 patients of the restriction group (78.4%) and 37 patients of the hydration group (86.0%), respectively. The frequency of catheterization was 1.3 0.7 times in the former and 1.6 0.7 times in the latter group. CONCLUSIONS: A strict restriction of fluid infusion appeared to be unnecessary for the purpose of preventing the urinary retention during surgery of benign anorectal diseases with spinal anesthesia.