Hypoglycemia results in cognitive dysfunction. The aim of this study was to assess and compare the degree of cognitive dysfunction experienced by insulin-dependent diabetic patients (IDDM) with hypoglycemia unawareness with patients with normal awareness of hypoglycemia. Cognitive function was examined in 10 patients who had normal awareness of the onset of hypoglycemia and 10 patients who had a history of impaired awareness of hypoglycemia. A hyperinsulinemic glucose clamp was used to manipulate blood glucose concentrations. Cognitive function was assessed using Rapid Visual Information Processing (RVIP), Trial Making B (TMB), Paced Auditory Serial Addition Test (PASAT) and Digit Symbol Substitution Test (DSST). Multivariate analysis of variance demonstrated a significant effect of hypoglycemia on cognitive function (p < 0.01). A trend was observed towards an overall effect of awareness on performance (p = 0.08). There were trends in the effects of awareness on RVIP correct responses across time (p = 0.07) and the interaction of awareness by study by TMB (p = 0.08). During hypoglycemia subjects with impaired awareness were less cautious in their responses (RVIP misses p = 0.03) and on recovery from hypoglycemia, their cognitive function remained abnormal (TMB p = 0.04, RVIP correct responses p = 0.02, RVIP misses p = 0.04). Thus, IDDM patients with hypoglycemia unawareness exhibited more profound cognitive dysfunction during acute hypoglycemia which persisted for longer following blood glucose recovery.