Citrulline (CIT), a non-protein amino acid in circulating blood, is almost exclusively contained in the enterocytes of small bowel mucosa and may represent a reliable marker of functioning enterocyte mass. The aim of this study was to evaluate the clinical utility of measuring serum citrulline levels in a group of patients affected by coeliac disease (CD). Fifty healthy volunteers, 21 patients with untreated coeliac disease and 6 patients with refractory coeliac disease took part in the study. Serum citrulline levels and duodenal lesions were evaluated at the time of diagnosis, and after at least 24 months of gluten-free diet. Serum citrulline concentrations were determined by ion exchange chromatography. In comparison to healthy volunteers, serum citrulline concentrations were significantly lower in untreated and refractory coeliac disease patients. No significant difference was found between untreated and refractory coeliac disease patients and between patients with different patterns of clinical presentation or various degrees of duodenal lesions. After a gluten-free diet, the mean of serum citrulline concentration was increased in all but one patient. Although, as expected, serum citrulline levels turned out to be low in coeliac disease, the clinical utility of their measurement is, at least, questionable in this condition.