OBJECTIVE - Diabetes is associated with cognitive decline and dementia. However, the relationship between the degree of hyperglycemia and cognitive status remains unclear. This was explored using baseline cognitive measures collected in the ongoing Memory in Diabetes (MIND) substudy of the Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial. RESEARCH DESIGN AND METHODS - The relationship of A1C and fasting plasma glucose (FPG) levels to performance on four cognitive tests was assessed, adjusting for age and other determinants of cognitive status. The tests were the Digit Symbol Substitution Test (DSST), Mini Mental Status Examination (MMSE), Rey Auditory Verbal Learning Test, and Stroop Test. RESULTS - A statistically significant age-adjusted association was observed between the A1C level and the score on all four cognitive tests. Specifically, a 1% higher A1C value was associated with a significant 1.75-point lower DSST score (95% CI -1.22 to -2.28; P < 0.0001), a 0.20-point lower MMSE score (-0.11 to -0.28; P < 0.0001), a 0.11-point lower memory score ( - 0.02 to -0.19, P = 0.0142), and a worse score (i.e., 0.75 s more) on the Stroop Test (1.31-0.19, P = 0.0094). The association between the DSST score and A1C persisted in all multiple linear regression models. FPG was not associated with test performance. © 2009 by the American Diabetes Association.