This study aimed to investigate the pattern of cellular proliferation and to calculate the growth fraction of small intestinal crypts in 24 patients--eight with untreated coeliac disease (CD), six with treated CD, and 10 controls with normal villous architecture. Duodenal biopsy specimens were stained with MI-B1 antibody using an immunoperoxidase method. Positive stained crypt cells were counted and their position within each intact crypt column was noted. These were used to construct proliferation index distribution curves for each cell position from which the crypt growth fraction for each group of patients was calculated. The crypt growth fraction was found to be higher in the untreated CD group (0.57) than in the treated patients (0.39) and the controls (0.44). The mean (SEM) proliferative index for each crypt was significantly higher in the untreated group (42.3 (10.8)%; p < 0.05) than in the treated CD (21.7 (8.5)%) and control (19.7 (7.5)%) groups and correlated closely with the surface:volume index and enterocyte cell height measurements in all three groups. An increased growth fraction may contribute to other changes in crypt cell kinetics to produce the histological changes seen in untreated CD.