It has not been studied yet whether factors such as the number of subjects recruited by specialized centers for multicenter trials may influence weight loss during a low-calorie diet (LCD). This study aimed at determining whether the number of recruited subjects per center might predict relative weight loss. This is a post hoc analysis of an existing database: 701 obese subjects (77% women, 23% men, mean BMI: 38.9 kg/m(2)) were enrolled at 22 sites (4-85 subjects/site) in five countries to follow a LCD providing 800-1,000 kcal/day during 8 weeks. The main outcome measure was the percentage weight loss after the 8-week LCD. Mean weight loss differed significantly between participating centers (5.8-11.8% of the initial weight; P <0.001). There was a significant positive correlation between relative weight loss and the number of recruited subjects per center (r = 0.38; P <0.001). In a multiple stepwise regression analysis, the number of recruited subjects per center appeared to be the main predictive factor of weight loss (R(2) = 0.07; P <0.001). As the number of participants within each center is clustered, we applied a hierarchical model to model the average weight loss vs. the number of participants included at each center. This model allows to predict that for 10 extra patients in a center, the average weight loss would increase by 0.5%. This is the first study suggesting that the number of recruited subjects per center may impact weight loss, and could therefore be considered as a new predictor for weight loss that is independent from the individual.