This prospective study investigated personality traits as predictors of the outcome in a 52-week weight-loss program for obesity. Personality traits were rated with the Inventory of Interpersonal Problems (IIP) in 54 consecutive obese outpatients when entering a multimodal weight-reduction program. The value of these variables to predict success was assessed after 12, 26, and 52 weeks of treatment. A stepwise linear regression analysis for all follow-ups was calculated to examine the impact of each IIP item subscale on weight reduction. The IIP item subscale "intrusive or needy" significantly correlated with weight reduction observed at every follow-up examination and accounts for 13.6% to 29.8% of the variance (p values < .05). Obese patients describing themselves as excessively friendly, outgoing, and sociable improved more from a weight-loss program than those with lower instances of these traits. Accordingly, personality traits deserve more attention at initial assessment and while planning treatment of obese patients.