Objective: The aim of this study was to identify the pre-treatment subject characteristics and weight loss changes as determinants of weight loss and attrition during a 10-week dietary intervention study. Methods: A total of 771 obese subjects (BMI 35.6 kg/m(2)) of both genders were included from 8 clinical centres in 7 European countries, who underwent a 10-week dietary intervention study comparing two hypo-energetic (-600 kcal/day) diets varying in fat content. Results: The multiple regression model showed that weight loss at week 10 was predicted by: 6.55 + 1.27 × early weight loss (kg) at week 1 + 1.35 × gender (R(2) = 0.28, p <0.001). When performing the corresponding model with half-way weight loss (week 5) as covariate, the regression equation was: weight loss (kg) at week 10 = 1.88 + 1.38 × half-way weight loss (kg) (week 5) + 0.42 × gender (R(2) = 0.77, p <0.001). A cut-off target of = 4 kg weight loss at week 5 emerged as an optimal predictor for reaching at least 10% weight loss at week 10. Greater attrition likelihood was predicted by high-fat diet, decreased early and half-way weight losses. Conclusion: Early and half-way weight losses are associated with and could contribute to prediction of the final weight loss and attrition. Copyright © 2012 S. Karger GmbH, Freiburg.