BACKGROUND: Physicians are encouraged to counsel overweight and obese patients to lose weight. PURPOSE: It was examined whether discussing weight and use of motivational interviewing techniques (e.g., collaborating, reflective listening) while discussing weight predicted weight loss 3 months after the encounter. METHODS: Forty primary care physicians and 461 of their overweight or obese patient visits were audio recorded between December 2006 and June 2008. Patient actual weight at the encounter and 3 months after the encounter (n=426); whether weight was discussed; physicians' use of motivational interviewing techniques; and patient, physician, and visit covariates (e.g., race, age, specialty) were assessed. This was an observational study and data were analyzed in April 2009. RESULTS: No differences in weight loss were found between patients whose physicians discussed weight or did not. Patients whose physicians used motivational interviewing-consistent techniques during weight-related discussions lost weight 3 months post-encounter; those whose physician used motivational interviewing-inconsistent techniques gained or maintained weight. The estimated difference in weight change between patients whose physician had a higher global motivational interviewing-Spirit score (e.g., collaborated with patient) and those whose physician had a lower score was 1.6 kg (95% CI=-2.9, -0.3, p=0.02). The same was true for patients whose physician used reflective statements: 0.9 kg (95% CI=-1.8, -0.1, p=0.03). Similarly, patients whose physicians expressed only motivational interviewing-consistent behaviors had a difference in weight change of 1.1 kg (95% CI=-2.3, 0.1, p=0.07) compared to those whose physician expressed only motivational interviewing-inconsistent behaviors (e.g., judging, confronting). CONCLUSIONS: In this observational study, use of motivational interviewing techniques during weight loss discussions predicted patient weight loss.