Increased gastrointestinal permeability was found in celiac disease already 20 years ago. Originally a genetical defect was supposed leading to elevated permeability and later to celiac disease. However, patients under long-term gluten-free diet normalized their permeability tests. On the other side, unaffected relatives of patients with celiac disease showed high permeability but not different from patients with irritable bowel syndrome and with a tendency for normalization during follow-up. In active symptomatic celiac disease, permeability is elevated in the gastroduodenum - measured by sucrose test - as well as in the small intestine - measured by lactulose/mannitol ratio. The sensitivity in active celiac disease is high (near 100%), the specificity is low due to high permeability in many intestinal diseases as in acute infectious gastroenteritis, Crohn's disease, nonsteroidal anti-inflammatory drug treatment, etc. In contrast, lactulose/mannitol test permeability is much less sensitive in silent celiac disease without diarrhea (74%). The real importance of permeability disease is established by its use for follow-up of celiac patients under gluten-free diet whereas it is correlated to the degree of mucosal atrophy. In vitro tests also show increased lactulose mucosal to serosal flux in celiac disease, but not correlated to oral permeability test. In conclusion, lactulose/mannitol test is the only noninvasive functional test in celiac disease which has essential importance in active celiac disease and in follow-up under diet.