This prospective, nonrandomized study was conducted to compare the increases in albumin and prealbumin concentrations in postoperative patients given adequate nutrition support. All surgery patients at least 18 years of age and who required parenteral nutrition were included. Of 86 patients evaluated, 16 met all criteria for study entry. Blood for albumin concentrations was drawn within 48 hours of beginning parenteral nutrition and then weekly. Blood for prealbumin concentrations was drawn within 48 hours of beginning parenteral nutrition and then twice weekly. Albumin concentrations increased from 2.00 +/- 0.35 to 2.21 +/- 0.42 g/dl (NS). Prealbumin concentrations increased from 11.97 +/- 6.31 to 17.29 +/- 8.93 mg/dl (p = 0.017). All but one prealbumin concentration was in the normal range for our laboratory when parenteral nutrition was discontinued. None of the albumin concentrations were ever in the normal range. The prealbumin concentration is a better indicator than albumin of nutrition status in the postoperative patient. Since prealbumin concentrations typically rise into the normal range within a week after adequate caloric supplementation, clinicians may avoid unnecessary increases in protein-calorie intake and laboratory testing of nutrition status by using this measurement.