This study assessed whether motivational interviewing (MI), applied as part of a standard guided self-help (GSH) behavioral weight loss treatment, reduced attrition rate and improved participant weight loss and other eating-related and general psychological measures, relative to a standard GSH treatment alone. Thirty-nine overweight patients (7 males, 32 females) were randomized to receive either 6 sessions of traditional guided self-help and 2 traditional motivation-focused sessions (GSH treatment); or 6 guided self-help sessions and 2 sessions utilizing an MI approach to motivation (GSH/MI treatment). In intention-to-treat analyses, significant differences were found in the areas of eating concern and control over eating, favoring GSH/MI. Between-group effect size estimates indicated a small advantage for GSH/MI over GSH in weight loss, as well as on a number of secondary eating-related measures. Implications and future directions are discussed.