A significant percentage of the patients admitted to our hospitals show signs of malnutrition that are neither detected nor treated and often become more severe during the hospital stay. This malnutrition can contribute to the loss of some important functions as, for instance the capacity for immune response and healing. In this study we use several simple, economical parameters to evaluate the nutritional status upon admittance and upon leaving the hospital of all the patients who in the course of a year occupied four randomly chosen beds in a general surgery ward at a county hospital in the Valencian Community. The results show that hospitalization reduced the number of over weight patients, duplicated the number of hypoalbuminemics and triplicated the number of people with body weight and body mass index (BMI) below normal levels. In patients over 40 hospitalization decrease the values of all the parameters studied, whereas in younger subjects only the weight, the BMI and serum albumin values were affected. Fifty seven percent of the patients suffer some kind of postoperative complication and this incidence is higher among patients with malnutrition upon admittance. The most common complication is the nosocomial pneumonia and urine infection. The nutritional parameters that best predict possible postoperative complications are low values of serum albumin, total lymphocyte count, tricipital fat skin fold and arm circumference. In general, postoperative complications show only a slight positive correlation with the length of the hospital stay, but the longer the stay is the worst the nutritional status becomes.