A previously described technique for measuring changes in the architecture of the small intestinal mucosa was used to measure the long-term response in six patients with adult coeliac disease to treatment with a gluten-free diet. All had flat or flat-with-mosaic jejunal mucosae before treatment and were subsequently believed to keep strictly to a gluten-free diet. They were followed for between four and 14 months from the start of treatment. The ratio of the area of surface epithelium to that of crypt epithelium (;area ratio') invariably increased following gluten withdrawal, but did not begin to do so until after several weeks of treatment. No patient attained complete histological normality within the limited period of observation, which was never more than 14 months. In one of the three patients followed for a year or more the area ratio increased to within the control range, and in two of these three the ratio was about 10 times the pretreatment ratio. There was a highly significant linear correlation between area ratio and time on the diet (r = 0.775, p < 0.005). The surface cell height measurements for these patients, by contrast, rose steeply and almost invariably during the first few weeks of treatment, but after that they fluctuated without further significant change. It is suggested that the two methods of measurement were complementary to each other, surface cell height being useful while the patient was in hospital on a rigidly strict diet, and the area ratio after return home, when minor dietary lapses were almost invariable.