Any surgical intervention is associated with an activation of protein catabolism, the extent of which is dependent on the severity of surgical trauma. There is a paucity of reports on protein catabolism after transplantation of chest organs (lung transplantation (LTX) and heart transplantation (HTX)). The aim of the present study was to quantify and compare the extent of postoperative protein catabolism and associated metabolic perturbations in patients after LTX and HTX. Eighteen consecutive patients after LTX and 15 consecutive patients after HTX who required postoperative intensive care for more than 4 days, constituted the study population. The nitrogen balance (assessed on the basis of the urea nitrogen production rate and nitrogen intake) was assessed retrospectively and correlated with insulin requirements, immunosuppression and the clinical course. Within the first 5 days the nitrogen balance became progressively negative in both groups, reaching a maximum on the 5th day. Thereafter the nitrogen balance of patients following LTX remained negative, whereas the nitrogen balance of patients following HTX tended to improve. The evolution of nitrogen balance significantly differed between both groups (p < 0.01). The mean nitrogen loss was -0.29 +/- 0.17g/kg BW/day after LTX versus -0.22 +/- 0.12g/kg BW/day after HTX. Smaller amounts of glucocorticoids were used for immunosuppression in patients after HTX than in patients after LTX; nevertheless, heart transplant recipients required higher doses of insulin to maintain normoglycemia. A regression analysis revealed that the duration of stay at the intensive care unit (p < 0.001) and the amount of glucocorticoids (p < 0.01) negatively affected the nitrogen balance, whereas an increased protein intake (p < 0.001) exerted a positive effect. Compared to other major surgical procedures, protein catabolism is excessively elevated in patients after thoracic transplantation. Immunosuppressive therapy with glucocorticoids contributes to protein degradation; the nitrogen balance after LTX is more negative than that after HTX because of higher glucocorticoid requirements following LTX. More aggressive nutritional intervention and especially an increased nitrogen intake might help to reduce protein losses in these patients.