BACKGROUND: Frequent self-weighing has been proposed as an adjuvant strategy to promote weight loss. Not all experts agree on its utility, and the literature supporting its effectiveness is somewhat limited by methodologic shortcomings related to the subjective assessment of self-weighing frequency. DESIGN: A prospective cohort design was utilized to examine 100 participants enrolled in a weight-loss trial that encouraged frequent, objectively measured self-weighing at home. Measurements were made at pretreatment and at follow-up visits at 6 and 12 months. SETTING/PARTICIPANTS: Participants were employed, obese adults enrolled in the Weigh By Day trial. Study data were collected between October 2005 and May 2007. INTERVENTION: The intervention consisted of a 6-month behavioral weight-loss program that employed telephone counseling, a written manual, and a home telemonitoring scale. MAIN OUTCOME MEASURES: The primary outcomes of interest were body weight and clinically meaningful weight loss (i.e., > or =5%). Analyses were performed in March 2008. RESULTS: Self-weighing was a significant predictor of body weight over time. Participants lost about 1 extra pound for every 11 days they self-weighed during treatment. In addition, participants who self-weighed at least weekly were 11 times more likely to lose at least 5% of their pretreatment weight after 6 months. Improvements attenuated after 12 months. CONCLUSIONS: Self-weighing may be a strategy to enhance behavioral weight-loss programs. Weekly self-weighing seems to be a reasonable, evidence-supported recommendation for successful weight loss, but more research is warranted to determine the independent contribution of self-weighing to successful weight loss, as well as its potential risk of negative psychological impact.