OBJECTIVES: To examine whether cognitive impairment in adults with diabetes mellitus is associated with worse glycemic control and to assess whether level of social support for diabetes mellitus care modifies this relationship. DESIGN: Cross-sectional analysis. SETTING: The 2003 Health and Retirement Study (HRS) Mail Survey on Diabetes and the 2004 wave of the HRS. PARTICIPANTS: Adults aged 50 and older with diabetes mellitus in the United States (N=1,097, mean age 69.2). MEASUREMENTS: Glycosylated hemoglobin (HbA1c) level; cognitive function, measured with the 35-point HRS cognitive scale (HRS-cog); sociodemographic variables; duration of diabetes mellitus; depressed mood; social support for diabetes mellitus care; self-reported knowledge of diabetes mellitus; treatments for diabetes mellitus; components of the Total Illness Burden Index related to diabetes mellitus; and functional limitations. RESULTS: In an ordered logistic regression model for the three ordinal levels of HbA1c (<7.0, 7.0?7.9, ?8.0 mg/dL), respondents with HRS-cog scores in the lowest quartile had significantly higher HbA1c levels than those in the highest cognitive quartile (adjusted odds ratio=1.80, 95% confidence interval=1.11?2.92). A high level of social support for diabetes mellitus care modified this association; for respondents in the lowest cognitive quartile, those with high levels of support had significantly lower odds of having higher HbA1c than those with low levels of support (1.11 vs 2.87, P=.02). CONCLUSION: Although cognitive impairment was associated with worse glycemic control, higher levels of social support for diabetes mellitus care ameliorated this negative relationship. Identifying the level of social support available to cognitively impaired adults with diabetes mellitus may help to target interventions for better glycemic control.