The effects of parenteral nutrition on appetite during and after therapy are unclear. Previous studies done in animals, as well as in humans, are inconclusive. The purpose of this study was to investigate the effects of parenteral nutrition on voluntary oral intake of food. The study was done on ten stable patients receiving parenteral nutrition for transient dysfunction of their gastrointestinal tract. For each patient, a calorie count of the ingested food was obtained for 3 consecutive days. Parenteral calories were then decreased without the patient's knowledge. A calorie count for 3 more days was obtained following the day of change. The mean daily oral intake was 823 kcal when the mean daily parenteral nutrition intake was 2,902, providing a total of 3,723 kcal. When parenteral nutrition calories were decreased to a mean of 1,550, the mean daily oral intake increased to 1,396. This difference in oral and parenteral calorie intake was statistically significant (P less than .001). It can be concluded from this data that parenteral nutrition decreases voluntary oral intake of food. It is therefore suggested that if the gastrointestinal tract is functionally satisfactory, parenteral nutrition can be rapidly weaned off, provided oral consumption is monitored to assure adequacy.