To evaluate the effect of case management by a clinical pharmacist on glycemic control and preventive measures in patients with type 2 diabetes mellitus. Randomized controlled trial in a university-affiliated primary care internal medicine clinic. We recruited 80 patients with poorly controlled type 2 diabetes mellitus. A clinical pharmacist provided evaluation and modification of pharmacotherapy, self-management diabetes education, and reinforcement of diabetes complications screening processes through clinic visits and telephone follow-up. The main clinical outcome was hemoglobin A1C (HbA1C) level; process measures included HbA1C and low-density lipoprotein measurement, retinal examination, urine microalbumin testing (or use of angiotensin-converting enzyme inhibitors), and monofilament screening for diabetic neuropathy. Patients in the intervention and control groups were similar in age, sex, mean HbA1C levels (10.1% and 10.2%, respectively; P = .65), and current treatment regimens at baseline. Patients who received case management by the clinical pharmacist achieved greater reduction in HbA1C levels than those in the control group (2.1% vs 0.9%, P = .03). Three of the 5 process measures were conducted more frequently in the intervention group than the control group, including low-density lipoprotein measurement (100.0% vs 85.7%, P = .02), retinal examination (97.3% vs 74.3%), and monofilament foot screening (92.3% vs 62.9%). Proactive diabetes case management by a pharmacist substantially improved glycemic control and diabetes process-of-care measures. This approach, integrated with and based in the primary care setting, was an effective and efficient approach to improving care, especially for those with poor glycemic control at baseline.