Objective measurements of various histological features in rectal biopsy material were made using computerised interactive image analysis. Analysis of the resulting data by forward stepwise discriminant analysis showed that biopsies from patients with ulcerative colitis and infective diarrhoea could be distinguished from each other and from normal biopsies; they could not, however, be distinguished from biopsies of patients with Crohn's disease, which behaves as a heterogeneous entity rather than a single disease. The measurements showed that the crypt atrophy described in active ulcerative colitis may be a misnomer, the appearances being due to an increase in mucosal depth and alteration in crypt configuration rather than a true atrophy. Unexpectedly, the ratio of crypt cell height to surface cell height gave the best separation of Crohn's disease from ulcerative colitis.