Background Mechanisms for racial/ethnic disparities in glycemic control are poorly understood. Methods A nationally representative sample of 1901 respondents 55 years or older with diabetes mellitus completed a mailed survey in 2003; 1233 respondents completed valid at-home hemoglobin A1c (HbA1c) kits. We constructed multivariate regression models with survey weights to examine racial/ethnic differences in HbA1c control and to explore the association of HbA1c level with sociodemographic and clinical factors, access to and quality of diabetes health care, and self-management behaviors and attitudes. Results There were no significant racial/ethnic differences in HbA1c levels in respondents not taking antihyperglycemic medications. In 1034 respondents taking medications, the mean HbA1c value (expressed as percentage of total hemoglobin) was 8.07% in black respondents and 8.14% in Latino respondents compared with 7.22% in white respondents (P < .001). Black respondents had worse medication adherence than white respondents, and Latino respondents had more diabetes-specific emotional distress (P < .001). Adjusting for hypothesized mechanisms accounted for 14.0% of the higher HbA1c levels in black respondents and 19.0% in Latinos, with the full model explaining 22.0% of the variance. Besides black and Latino ethnicity, only insulin use (P < .001), age younger than 65 years (P = .007), longer diabetes duration (P = .004), and lower self-reported medication adherence (P = .04) were independently associated with higher HbA1c levels. Conclusions Latino and African American respondents had worse glycemic control than white respondents. Socioeconomic, clinical, health care, and self-management measures explained approximately a fifth of the HbA1c differences. One potentially modifiable factor for which there were racial disparities–medication adherence–was among the most significant independent predictors of glycemic control.