We studied the possible clinical significance of high basal levels of H2 by analyzing the breath excreted by the following fasting subjects: 50 healthy volunteers, 149 subjects with functional bowel disorders, 16 patients with small bowel bacterial overgrowth proven by bacteriology, 34 patients with untreated celiac disease, 40 patients with celiac disease on a gluten-free diet, and 40 patients with disorders of the small intestine other than celiac disease (disease controls). The fasting levels of H2 in untreated celiac patients (mean 22.5 +/- 19.3 ppm) were significantly higher than those in healthy volunteers (5.8 +/- 3.1 ppm), patients with functional bowel disorders (6.6 +/- 4.4 ppm), treated celiac patients (9.9 +/- 8.1 ppm), and disease controls (7.0 +/- 6.7 ppm). No significant difference was found between patients with untreated celiac disease and bacterial overgrowth (mean 14.7 +/- 14.0 ppm). The percentage of patients with elevated H2 fasting levels in untreated celiac disease (58.8%) was significantly higher than that in the other groups, except for the patients with bacterial overgrowth (43.7%). In 14 celiac patients, studied before and after a gluten-free diet, fasting H2 levels decreased from 26.6 +/- 18 to 11.6 +/- 10 ppm, becoming normal only in those patients with healing of intestinal lesions. Our results show that high fasting H2 levels are a frequent feature of untreated celiac disease and that the return to normal of these levels is predictive of villous regrowth.