Patients undergoing bariatric surgery with a gastric bypass lose about 66% of excess weight. Although this procedure induces weight loss, it is unknown whether it leads to protein malnutrition, which is studied here. One hundred ten obese patients (body mass index, 47.9 +/- 8.6 kg/m(2)) undergoing gastric bypass had a measurement of plasma albumin and transthyretin (formerly prealbumin) and a calculation of nutritional risk index (NRI) before and throughout the 2 years following the surgery. All but five patients lost more than 15% of initial weight; the mean loss of excess weight was 65.2 +/- 26.4% at 2 years. Plasma concentrations of albumin and transthyretin decreased after surgery, but while albumin returned to initial values after 12 months, transthyretin remained low. Only one patient had an albumin below 30 g/l; another one had a transthyretin lower than 110 mg/l. All NRI scores were lower than 83.5 (62 +/- 5, ranging 44-70), qualifying patients for severe malnutrition. Malnutrition is difficult to diagnose in obese patients undergoing surgery. The large weight loss is most often not associated with protein malnutrition. Whether gastric bypass induces protein malnutrition remains to be established.