Most primary care providers (PCPs), constrained by time and resources, cannot provide intensive behavioral counseling for obesity. This study evaluated the effect of using medical assistants (MAs) as weight loss counselors. The study was a randomized controlled trial conducted in two primary care offices at an academic medical center. Patients (n = 50) had a BMI of 27-50 kg/m(2) and no contraindications to weight loss. They were randomized to quarterly PCP visits and weight loss materials (Control group) or to the same approach combined with eight visits with a MA over 6 months (Brief Counseling). Outcomes included change in weight and cardiovascular risk factors (glucose, lipids, blood pressure, and waist circumference). Patients in the Brief Counseling and Control groups lost 4.4 +/- 0.6 kg (5.1 +/- 0.7% of initial weight) and 0.9 +/- 0.6 kg (1.0 +/- 0.7%), respectively, at month 6 (P < 0.001). There were no significant differences between groups for changes in cardiovascular risk factors. Brief Counseling patients regained weight between month 6 and month 12, when MA visits were discontinued. Attrition was 10% after 6 months and 6% after 12 months. Brief Counseling by MAs induced significant weight loss during 6 months. Office-based obesity treatment should be tested in larger trials and should include weight loss maintenance counseling.