In anorexia nervosa, psychopathological features and reduced body weight are inseparable, suggesting a prominent role of behavioral factors in achievement and maintenance of extreme underweight. Due to the considerably higher prevalence of this eating disorder in females, anorexia nervosa contributes to the left end of the distribution of the body mass index, especially in the female sex. By reviewing the relevant literature we examined whether genetic research in anorexia nervosa can profit from considering this disorder as an extreme weight condition. For this purpose we compared genetic studies pertaining to both anorexia nervosa and the heritability of the body mass index. Whereas previous genetic studies in anorexia nervosa have mostly concentrated on the assessment of the familial psychopathology, further studies are warranted that additionally attempt to analyze the complex phenotype body weight in relatives of affected probands. Further insight into pathogenetic mechanisms underlying anorexia nervosa might be gained by contrasting the epidemiological, psychopathological and prognostic factors with those in severe obesity. Thus, epidemiological studies suggest that females are more likely to develop both extreme underweight and extreme obesity. A possible explanation for this phenomenon is that the, on average, higher percentage of total body weight composed of fat mass might predispose females towards the development of both extreme weight conditions.