OBJECTIVES: To investigate the impact of comorbid cardiovascular disease (CVD) on antidiabetic medication adherence.  METHODS: Eligibility and claims data (2002-2004) were used to identify patients ≥40 years of age with a diagnosis of type 2 diabetes concurrent with hypertension (HYPT), coronary artery disease (CAD), and/or heart failure (HF), and at least two prescription fills for an antidiabetic medication. Medication adherence was assessed using proportion of days covered ≥0.8. Multivariable logistic regressions were used to assess CVD and other risk factors associated with nonadherence with antidiabetic medication using 2004 data.  RESULTS: A total of 16,922 patients were identified.  Patients with two comorbid CVD were more likely to use or be adherent with combination cardiovascular and antidiabetic medications [HYPT+CAD (79%/28%, represented as proportion of use/adherent rate), HF+CAD (88%/31%)] than those with a single comorbid CVD [HYPT (68%/21%), CAD (65%/21%), and HF (72%/21%), respectively, p<0.0001].  The adherence rate for use of both antidiabetic and cardiovascular medications was only 24%.  The major significant predictors of diabetic medication nonadherence included no fill of (OR:2.62, 95% CI:2.28-3.01) or nonadherent with (OR:3.43, CI:3.13-3.75) cardiovascular medication; MediCal-only eligibility (OR:1.76, CI:1.62-1.91 vs. MediCal-Medicare eligibility); nonompliance with diabetes care guidelines (no eye examination, no LDL test, and less than two HbA1c tests during 2004) (OR:1.48, CI:1.37-1.59); a greater number of inpatient (OR:1.17, CI:1.10-1.24) or diabetes-related inpatient visits (OR:1.41, CI:1.06-1.87); Black race (OR=1.47, CI: 1.29-1.66 vs. White); type of comorbid CVD vs. HF+CAD [HYPT (OR:0.73, CI:0.65-0.81), CAD (OR:0.73, CI:0.67-0.86), HYPT+CAD (OR:0.76, CI:0.67-0.86)]; two or more cardiovascular medication fills (OR:1.35, CI:1.18-1.55).  CONCLUSIONS: CVD comorbidity and nonadherence with cardiovascular medications and diabetes care guidelines were major significant factors associated with nonadherence to antidiabetic medications in a California Medicaid sample.  Patient nonadherence behaviors should be considered when providing care for diabetes patients with comorbid CVD.