Small bowel functions are more complex than colon functions, and short bowel conditions are associated with increased mortality. Gastrointestinal dysfunction in critically ill patients is common, probably underestimated, and associated with a poor prognosis. However, a validated definition of acute intestinal failure is lacking, in absence of a marker to measure it. Consequently, small bowel dysfunction is not clearly integrated into the overall approach used to treat ICU patients. Review of the literature on gastrointestinal dysfunction in critically ill patients, and proposition of a definition of acute intestinal failure. On the one hand, small bowel ischemia is related to acute reduction of enterocyte mass and loss of gut barrier function by epithelial lifting of villi. On the other hand, systemic inflammatory response syndrome (SIRS) and sepsis could be linked to an acute dysfunction of enterocytes without enterocyte reduction. Citrulline is an amino acid mainly synthesized by small bowel enterocytes. Various contexts of chronic and acute reduction of enterocyte mass have been correlated with low plasma citrulline concentration. Critically ill patients with shock have an acute reduction of enterocyte mass and reduced gut citrulline synthesis, leading to a low plasma citrulline concentration. Acute intestinal failure could be defined as an acute reduction of enterocyte mass and/or acute dysfunction of enterocytes, associated or not with loss of gut barrier function. The influence of SIRS and acute renal failure on plasma citrulline concentration and the value of this concentration as an indicator of acute intestinal failure in critically ill patients must be further evaluated. © Copyright jointly held by Springer and ESICM 2011