SUMMARY A group of teenage coeliac patients has been followed at three monthly intervals in the Outpatient Department to assess their progress and also to monitor their ability to maintain a gluten-free diet. After a follow-up period of four to six years a detailed reassessment was carried out in hospital on 10 patients, only five of whom had persevered with a gluten-free diet. The jejunal mucosal histology of those patients who did not persist with a gluten-free diet remained 'flat' although these patients appeared to have remitted clinically. Those subjects who did persist with a gluten-free regime had a normal or near normal mucosal histology. It was difficult on the basis of clinical, haematological, or biochemical criteria to separate the two groups. The best single assessment of whether these patients were maintaining a gluten-free diet was serial serum folate estimations. It is often extremely difficult to say whether teenage coeliac patients are keeping to their diet unless repeated jejunal biopsies are obtained, and this study suggests that serial serum folate estimations can act as a reasonable criterion of whether subjects are maintaining a gluten-free regime.It is now apparent that the sensitivity of the jejunal mucosa to gluten in coeliac patients is not confined to the paediatric period but is probably present throughout life (Sheldon and Tempany, 1966;Mortimer, Stewart, Norman, and Booth, 1968;Sheldon, 1969). Accordingly it is in the best interest of these patients that they should remain on a glutenfree diet both from the point of view of their general health and because the 'flat' intestinal mucosa which results from a gluten challenge in these subjects is a premalignant lesion (Gough, Read, and Naish, 1962;Harris, Cooke, Thompson, and Waterhouse, 1966;Austad, Comes, Gough, McCarthy, and Read, 1967;Brunt, Sircus, and MacLean, 1969). It is probable that withdrawing gluten from the diet on a permanent basis will protect them from the tendency to malignant change (Harris, Cooke, Thompson, and Waterhouse, 1967