Weight and body mass index are used in clinical routine to detect malnourished patients. However, a study performed at the emergency admission of the University Hospital of Geneva shows that the prevalence of malnutrition is very underestimated when based on body mass index instead of fat-free mass. Indeed, a third of the patients with normal body mass index, i.e. between 20 and 24.9 kg/m2, present a fat-free mass under percentile 10. This suggests that the body mass index alone cannot be used as diagnostic criteria for malnutrition. This review describes the prevalence of malnutrition at the emergency admission of the University Hospital of Geneva, the prevalence of malnutrition in the literature and its relationship with length of hospital stay and mortality.