The prevalence of the small intestine bacterial overgrowth syndrome has been assessed in 109 in-patients affected by various gastrointestinal disorders using the 1 g [14C]-xylose breath test; 18 healthy subjects acted as a control group: none of them showed abnormal results (100% specificity). None of 14 patients with colonic disease had abnormal results, whereas in 44 patients with ileal diseases the test was positive in 12% to 39% of the cases. Abnormal results were found in 46% of patients who underwent partial gastric resection greater than 20 years before, 29% of patients with irritable bowel syndrome without diarrhoea (faecal wet weight less than or equal to 600 g/72 h), 56% of those with diarrhoea of obscure origin, and 25% of celiac patients. Eight out of 8 patients with altered results showed normalization of the test after antibiotic therapy. Despite its high diagnostic value in the setting of clinical research, the 1 g [14C]-xylose breath test cannot as yet be proposed as a routine investigation.