Objective Predicting outcome in weight loss trials from baseline characteristics has proved difficult. Readiness to change is typically measured by self-report. Methods Performance of a behavioral task, completion of food records, from the screening period in the Look AHEAD study (n = 549 at four clinical centers) was assessed. Completeness of records was measured by the number of words and Arabic numerals (numbers) recorded per day, the number of eating episodes per day, and days per week where physical activity was noted. The primary outcome was weight loss at one year. Results In univariable analysis, both the number of words recorded and the number of numbers recorded were associated with greater weight loss. In multivariable analysis, individuals who recorded 20-26, 27-33, and ≥34 words per day lost 9.12%, 11.40%, and 12.08% of initial weight, compared to 8.98% for individuals who recorded less than 20 words per day (P values of 0.87, 0.008, and <0.001, respectively, compared to <20 words per day). Conclusions Participants who kept more detailed food records at screening lost more weight after 1 year than individuals who kept sparser records. The use of objective behavioral screening tools may improve the assessment of weight loss readiness.