BACKGROUND: A significant proportion of US Latinos with diabetes have limited English proficiency (LEP). Whether language barriers in health care contribute to poor glycemic control is unknown. OBJECTIVE: To assess the association between limited English proficiency (LEP) and glycemic control and whether this association is modified by having a language-concordant physician. DESIGN: Cross-sectional, observational study using data from the 2005-2006 Diabetes Study of Northern California (DISTANCE). Patients received care in a managed care setting with interpreter services and self-reported their English language ability and the Spanish language ability of their physician. Outcome was poor glycemic control (glycosylated hemoglobin A1c > 9%). KEY RESULTS: The unadjusted percentage of patients with poor glycemic control was similar among Latino patients with LEP (n = 510) and Latino English-speakers (n = 2,683), and higher in both groups than in whites (n = 3,545) (21% vs 18% vs. 10%, p < 0.005). This relationship differed significantly by patient-provider language concordance (p < 0.01 for interaction). LEP patients with language-discordant physicians (n = 115) were more likely than LEP patients with language-concordant physicians (n = 137) to have poor glycemic control (27.8% vs 16.1% p = 0.02). After controlling for potential demographic and clinical confounders, LEP Latinos with language-concordant physicians had similar odds of poor glycemic control as Latino English speakers (OR 0.89; CI 0.53-1.49), whereas LEP Latinos with language-discordant physicians had greater odds of poor control than Latino English speakers (OR 1.76; CI 1.04-2.97). Among LEP Latinos, having a language discordant physician was associated with significantly poorer glycemic control (OR 1.98; CI 1.03-3.80). CONCLUSIONS: Language barriers contribute to health disparities among Latinos with diabetes. Limited English proficiency is an independent predictor for poor glycemic control among insured US Latinos with diabetes, an association not observed when care is provided by language-concordant physicians. Future research should determine if strategies to increase language-concordant care improve glycemic control among US Latinos with LEP.