A citrulline threshold of 20 µmol/L apparently permitted the classification of short bowel syndrome patients into either transient or permanent intestinal failure categories, with 92% sensitivity, 90% specificity, and 95% positive and 86% negative predictive values, respectively. Experimental studies have been carried out also in assessing the value of citrulline as a marker for severity of small bowel epithelial damage from radiation. The plasma citrulline was shown to be a simple, non-invasive and sensitive assay to monitor and quantify radiation-induced small bowel damage in mice and humans. Otherwise, the literature on citrulline as a potential marker of intestinal and nutritional integrity is young and data for specific conditions come only from single centres; there are limited data on normal ranges. More crucially, however, there has been no attempt to clarify the effect of inflammation on citrulline homeostasis. To date there is no information in respect of patients with intestinal failure in whom there has been no resection.