Background. This study seeks to identify the genetic and environmental risk factors for the overvalued ideas that are characteristic of bulimia nervosa, using a biometrical model fitting approach with twin data. Methods. The Eating Disorder Examination (EDE), which can be used to gain continuous measures of dietary restraint, eating concern, weight concern and shape concern, was administered to 325 female twins, both monozygotic (MZ) and dizygotic (DZ). For each subscale, questions were asked concerning the month prior to interview and lifetime prevalence ('ever'). Results. Model fitting indicated that there is a powerful role of the environment in shaping women's attitude towards weight, shape, eating and food, ranging from 38% to 100% of the variance. For all subscales, with the exception of weight concern, the best explanation for individual variation was one that incorporated additive genetic and non-shared environmental influences. In contrast, model fitting indicated that non-shared and shared environmental influences best explained the variance of weight concern. Conclusions. With the exception of the Shape Concern subscale, environmental factors make a greater contribution than genetic factors to the development of the overvalued ideas that are seen to be one of the triggers for the development of bulimia nervosa. Given this substantial role of the environment influences, it seems likely that environmental manipulation can be effective in the prevention of bulimia nervosa.