OBJECTIVES: The aim of this retrospective study is to evaluate the adherence of type 2 diabetes (T2DM) patients starting drug treatment to the ADA guidelines on HbA1Ctesting and treatment modification and determine its impact on treatment outcomes. METHODS:    Data was obtained from a large health care plan claims database between July-2008 to December-2011. Eligible patients were aged ≥18 years with ≥2 T2DM diagnoses (ICD-9CM codes 250.x0, 250.x2), and were drug-naïve for ≥6 months before the first antidiabetic drug (termed “index treatment”) was required. Patient adherence to the HbA1C testing guideline was defined as an initial HbA1C test within 105 days of the index treatment and subsequent tests within 105 or 195 days of the previous test depending on the result (≥7 or <7%, respectively). Adherence to the drug modification guideline was defined as a change in treatment within 45 days of HbA1C ≥7%. Patient outcome after one year was evaluated using the HbA1Cvalues closest to 365 days after index treatment. RESULTS:   Of the 14,164 patients who met the study criteria, 4,419(31.20%) met the testing criteria for drug modification (HbA1C ≥7%). Of these patients, 546(12.36%) met the recommended testing frequency, 934(21.14%) adhered to the drug modification guidelines, and 117(2.65%) met both guidelines. The odds ratio of a patient achieving the HbA1C target (<7%) who adhered to the testing guideline was 4.66 compared with a patient who did not meet the testing guideline. Furthermore, the odds ratio of a patient in this population achieving the HbA1Ctarget was 4.95 when both guidelines were met, as compared with a patient who met neither guideline. CONCLUSIONS: Adherence to ADA guidelines on HbA1C testing frequency and therapy modification correlated with improved outcomes one year after initial drug treatment.