As an integral part of the United States health care delivery system, pharmacists are ideally positioned to provide diabetes mellitus management. In this systematic review, we assessed the effectiveness of diabetes quality improvement strategies delivered by pharmacists in outpatient settings. Five electronic databases were searched for articles published through August 2007. Only randomized controlled trials, controlled clinical trials, or cohort studies with a control group were eligible for inclusion. All interventions targeted adults with type 1 or 2 diabetes and measured diabetes-related complications and/or surrogate outcomes of hemoglobin A1c (A1C), blood pressure, or lipid profiles. Study data were abstracted by one author and checked by a second author following a standardized template. Results were synthesized narratively, and study-specific effect measures were calculated for the outcomes of interest. Twenty-one articles met the inclusion criteria: nine randomized controlled trials, one controlled clinical trial, and 11 cohort studies. All interventions involved additional visits by pharmacists with expanded roles to care for adult patients with diabetes. The A1C was the primary outcome of interest for all but two studies. Results of this review revealed overall improvement in A1C for patients in a diverse group of settings and across multiple study designs. Studies with smaller numbers of participants and those performed in the United States generally showed greater improvements in intervention group measures of A1C. A greater effect was also noted when pharmacists were afforded prescriptive authority. Only a few studies examined health care resource use; their results suggested that pharmacist interventions can reduce long-term costs by improving glycemic control and thus diminishing future diabetes complications. Findings from this review are limited by flaws in the study designs, including the high potential for selection bias in the study populations. However, due to the clinical significance of reported improvements in A1C, further trials with pharmacist case managers are warranted. Prospective assessments of the comparable efficacy of pharmacists to improve diabetes outcomes through self-management education and pharmacologic management are recommended.