Assessment of the nutritional status of surgical patients with a mean weight loss of 11.5 kg included determination of the following biochemical parameters: prealbumin, retinol-binding protein (RBP), serum vitamin A and serum zinc. Patients' immunological status was investigated by delayed hypersensitivity skin tests. Prealbumin and RBP are decreased in patients. RBP levels are significantly lower than in controls: RBP therefore seems to be a better criterion of denutrition than prealbumin. Serum RBP and vitamin A concentrations are not always correlated in patients; for instance, in malabsorptions, the sharp fall in serum vitamin A is isolated. The decrease in mean serum zinc concentrations is minor. Three patients have positive skin tests with severe biochemical disturbances; only serum zinc concentrations are normal and correlated with the results of skin tests. Interpretation of investigations in surgical patients is dependent on the cause of weight loss. However, biochemical and immunological assessment of the nutritional status can demonstrate specific deficits and improve preoperative nutrition.