OBJECTIVE To directly examine whether hypoglycemia differentially slows cognitive versus motor function, to evaluate the reliability of hypoglycemic-related slowing, and to examine factors contributing to individual differences. RESEARCH DESIGN AND METHODS IDDM subjects (n = 10) were administered a pure cognitive and a pure motor neuropsychological test at euglycemia (5.4 mmol), blood glucose nadir (2.6 mmol), postnadir (3.6 mmol), and again at euglycemia (6.7 mmol). To assess the practice effect, matched control subjects were tested at similar time intervals. RESULTS Concurrent and test-retest reliability for all tests was robust (r = 0.68–0.94). Only cognitive tasks demonstrated impairment at nadir (P < 0.04). Individual differences, in terms of cognitive impairment, were significantly correlated with levels of blood glucose at nadir and baseline performance. CONCLUSIONS Cognitive tasks appear to be more sensitive to neuroglycopenia than motor tasks. Cognitive impairment caused by hypoglycemia is reliable and differs across subjects. Individuals who show reliable sensitivity to cognitive impairments of hypoglycemia should avoid moderately low blood glucose levels.