The authors report four cases of patients presenting with hyperlactatemia following renal transplantation. The post-transplantation course of three patients who underwent renal transplantation was complicated by occult haemorrhage. Excessive blood loss was not evident, the patients were haemodynamically stable and their blood pressure, pulse rare and filling pressure (central venous pressure) were unremarkable. Late examinations revealed an early increase in arterial lactate concentrations a non-aniograp acidosis or lactic acidaemia occurred. Surgical decompression was carried on in all patients. An increase in the intraabdominal pressure might have caused renal impairment in the absence of haemodynamic disturbance, and retroperitoneal haematoma a change in the distribution of intrarenal blood flow. Another patient developed a partial renal venous thrombosis associated with hyperlactatemia. During this re-operation, a renal lactate production was measured. The renal cortex is a site of lactate clearance. Impaired renal perfusion should result in decreased lactate clearance and when the kidney is hypoperfused a lactate production was occur. In the absence of any signs of clinical shock, patient at risk of retroperitoneal haematoma or presenting with oliguria should benefit from lactate measurements, which could help diagnosing severe hypoperfusion of the graft.