To investigate the effect of estrogen deficiency on cognitive function in surgically menopausal women, a prospective study was conducted at the University Hospital in Assiut, Egypt, during the period of July 1997 to August 1999. The study included 35 women subjected to total abdominal hysterectomy and bilateral salpingo-oophorectomy for nonmalignant causes. They were subjected to cognitive assessment by Mini-Mental State Examination (MMSE), Wechsler Memory Scale (WMS) subtests, and measurement of auditory Event-Related Potentials (ERPs) and serial serum estradiol levels determination. Eighteen age- and education-, body-weight- and parity-matched control women were recruited for comparison. A significant decline in MMSE, WMS subtests (digit span, visual memory, logical memory and mental control) and prolongation of P300 of ERP latency was observed in the patient group at 3 and 6 months postoperatively. These changes were not observed in the control group. A significant correlation was found between serum estradiol level and mental control subtest score and P300 latency in patients preoperatively. Patients who had a drop of estrogen level >50% had more cognitive function decline. Rapid decline in estrogen level following surgical menopause was associated with a deleterious effect on cognitive function. Such observations may contribute to more understanding of the age-related cognitive decline in females.