OBJECTIVE: To hypothesize that patients with comorbid depression and diabetes and poor disease control will have poorer adherence to disease control medication and less likelihood of physician intensification of treatment. Many patients with diabetes fail to achieve American Diabetes Association Guidelines for glycemic, blood pressure and lipid control. Depression is a common comorbidity and may affect disease control through adverse effects on adherence and physician intensification of treatment. METHODS: In a cohort of 4117 patients with diabetes, depression was measured at baseline with the Patient Health Questionnaire-9 (PHQ-9). Patient adherence and physician intensification of treatment were measured in those who had evidence of poor disease control (HbA(1c) >or=8.0%, LDL >or=130 mg/dL, systolic blood pressure >or=140 mm Hg) over this 5-year period. Poor adherence was defined as having medication refill gaps for >or=20% of days covered for medications prescribed for each of these conditions. Treatment intensification was defined as an increased medication dosage in a class, an increase in the number of medication classes, or a switch to a different class within 3-month periods before and after notation of above target levels. RESULTS: Among patients with diabetes and poor disease control, depression was associated with an increased likelihood of poor adherence to diabetes control medications (odds ratio [OR] = 1.98; 95% Confidence Interval [CI] = 1.31, 2.98), antihypertensives (OR = 2.06; 95% CI = 1.47, 2.88), and LDL control medications (OR = 2.43; 95% CI = 1.19, 4.97). In patients with poor disease control who were adherent to medication or not yet started on a medication, depression was not associated with differences in likelihood of physician intensification of treatment. CONCLUSIONS: In patients with diabetes and poor disease control, depression is an important risk factor for poor patient adherence to medications, but not lack of treatment intensification by physicians.