This study compared the postoperative mental function in 44 elderly patients following general anaesthesia (GA) or spinal anaesthesia (SA) with sedation for transurethral resection of prostate. The Mini-Mental State (MMS) was done preoperatively and postoperatively at six hours, one day, three days, five days and one month. The geriatric mental status examination was performed preoperatively and one month after the anaesthetic. There was no significant intergroup difference in the MMS score in the preoperative, six hours, one day, three days, five days and 30 days postoperative scores between the GA and SA with sedation groups. A significant intragroup difference between preoperative and postoperative MMS score was detected in the GA group (P less than 0.02) and in the SA group with sedation (P less than 0.03). In the GA group, the significant decrease in MMS score occurred at 6 h postoperatively (P less than 0.002) whereas in the SA group with sedation, MMS score also decreased significantly at 6 h (P less than 0.005). In conclusion, there was no significant difference in perioperative mental function between the general and spinal anaesthetic groups when supplemental IV sedation was given. In both groups, perioperative mental function decreased significantly at 6 h postoperatively.