OBJECTIVE: To investigate the relationship between recurrent hypoglycemia and cognitive impairment in insulin-dependent diabetic patients. RESEARCH DESIGN AND METHODS: Seventy patients who were diagnosed as diabetic at age 18 years or older, were under 55 years old, and had no condition likely to affect cognitive abilities were recruited from a diabetic register. Patients were interviewed to obtain information on the frequency of major and minor hypoglycemia. Their cognitive abilities were assessed on tests of premorbid intelligence, current intelligence, reaction time, concentration, memory, and information processing. RESULTS: There was a significant correlation between the apparent decline in intelligence, expressed as the discrepancy between the estimated premorbid and the actual performance intelligence quotient, and the frequency of major hypoglycemic attacks (rs = −0.30; P < 0.01). Comparison of patients with and without recurrent hypoglycemia showed few significant differences in cognitive ability. CONCLUSIONS: Results support previous work that suggests that major hypoglycemic attacks have a significant effect on some aspects of cognitive function, but the clinical importance of this finding remains to be determined.