BACKGROUND: Depression frequently occurs in combination with diabetes mellitus, adversely affecting the course of illness. OBJECTIVE: To determine whether enhancing care for depression improves affective and diabetic outcomes in older adults with diabetes and depression. DESIGN: Preplanned subgroup analysis of the Improving Mood-Promoting Access to Collaborative Treatment (IMPACT) randomized, controlled trial. SETTING: 18 primary care clinics from 8 health care organizations in 5 states. PATIENTS: 1801 patients 60 years of age or older with depression; 417 had coexisting diabetes mellitus. INTERVENTION: A care manager offered education, problem-solving treatment, or support for antidepressant management by the patient’s primary care physician; diabetes care was not specifically enhanced. MEASUREMENTS: Assessments at baseline and at 3, 6, and 12 months for depression, functional impairment, and diabetes self-care behaviors. Hemoglobin A(1c) levels were obtained for 293 patients at baseline and at 6 and 12 months. RESULTS: At 12 months, diabetic patients who were assigned to intervention had less severe depression (range, 0 to 4 on a checklist of 20 depression items; between-group difference, -0.43 [95% CI, -0.57 to -0.29]; P < 0.001) and greater improvement in overall functioning (range, 0 [none] to 10 [unable to perform activities]; between-group difference, -0.89 [CI, -1.46 to -0.32]) than did participants who received usual care. In the intervention group, weekly exercise days increased (between-group difference, 0.50 day [CI, 0.12 to 0.89 day]; P = 0.001); other self-care behaviors were not affected. At baseline, mean (+/-SD) hemoglobin A1c levels were 7.28% +/- 1.43%; follow-up values were unaffected by the intervention (P > 0.2). LIMITATIONS: Because patients had good glycemic control at baseline, power to detect small but clinically important improvements in glycemic control was limited. CONCLUSIONS: Collaborative care improves affective and functional status in older patients with depression and diabetes; however, among patients with good glycemic control, such care minimally affects diabetes-specific outcomes.