The present study attempted to predict weight loss in, and completion of, low-calorie diet (LCD, n=167) and very low-calorie diet (VLCD, n=96) weight loss programs. Program completion and weight loss were examined in association with three groups of variables: demographic (e.g., age), medical/physical (e.g., body mass index [BMI]), and motivational/behavioral (e.g., exercise, ratings for motivation and commitment for the program). In the LCD group, age was positively associated with program completion, and commitment to the program was negatively associated with both program completion and weight loss. In the VLCD group, no variables predicted program completion, and only male gender predicted weight loss. Our results offer little to contradict previous findings that preexisting participant characteristics are of limited utility in predicting weight loss treatment outcome. In the LCD group, the negative associations of commitment with program completion and weight loss were surprising. If replicated, they might suggest the need to clarify patients' expectations about the level of commitment required for successful treatment.