OBJECTIVES: To determine whether a home-based multicomponent physical activity counseling (PAC) intervention is effective in reducing glycemic measures in older outpatients with prediabetes mellitus. DESIGN: Controlled clinical trial. SETTING: Primary care clinics of the Durham Veterans Affairs (VA) Medical Center between September 29, 2008, and March 25, 2010. PARTICIPANTS: Three hundred two overweight (body mass index 25-45 kg/m(2) ), older (60-89) outpatients with impaired glucose tolerance (fasting blood glucose 100-125 mg/dL, glycosylated hemoglobin (HbA1c) <7%) randomly assigned to a PAC intervention group (n = 180) or a usual care control group (n = 122). INTERVENTION: A 12-month, home-based multicomponent PAC program including one in-person baseline counseling session, regular telephone counseling, physician endorsement in clinic with monthly automated encouragement, and customized mailed materials. All study participants, including controls, received a consultation in a VA weight management program. MEASUREMENTS: The primary outcome was a homeostasis model assessment of insulin resistance (HOMA-IR), calculated from fasting insulin and glucose levels at baseline and 3 and 12 months. HbA1c was the secondary indicator of glycemic control. Other secondary outcomes were anthropometric measures and self-reported physical activity, health-related quality of life, and physical function. RESULTS: There were no significant differences between the PAC and control groups over time for any of the glycemic indicators. Both groups had small declines over time of approximately 6% in fasting blood glucose (P < .001), and other glycemic indicators remained stable. The declines in glucose were not sufficient to affect the change in HOMA-IR scores due to fluctuations in insulin over time. Endurance physical activity increased significantly in the PAC group (P < .001) and not in the usual care group. CONCLUSION: Home-based telephone counseling increased physical activity levels but was insufficient to improve glycemic indicators in older outpatients with prediabetes mellitus.