Attrition in weight loss programs is a complex process, influenced by patients' pretreatment characteristics and treatment variables, but available data are contradictory. Only a few variables have been confirmed by more than one study as relevant risk factors, but recently new data of clinical utility emerged from "real world" large observational studies. Practical difficulties, unsatisfactory results, a few psychological variables (e.g. lack of motivation, unrealistic weight loss expectations, overall level of stress, self-confidence in the ability to lose additional weight without professional help, and sense of abandonment from therapists), seem to play a crucial role in the patients' decision of interrupting the weight loss treatment. (Contains 3 tables.)