The aim of this study was to clear up, whether in adults morphometric investigation of rectal biopsies could improve the differential diagnosis of Crohn's disease and ulcerative colitis. Rectal biopsies were available of 63 normal controls, of 65 patients suffering from Crohn's disease and 72 patients with ulcerative colitis. The serially cut biopsies were investigated histologically both by morphometric methods and by subjective rules of daily practice in five randomly chosen sections. The data were evaluated by multivariate discriminate analysis. Morphometric analysis on the basis of 16 variables brought a significantly better discrimination between Crohn's disease and ulcerative colitis than evaluation by means of 5 criteria of conventional daily routine. This improved discrimination, however, is at least partially produced by incidental variations between the subsets under study. Therefore, the functions of our multivariate discriminate analyses cannot be used for differentiation of chronic inflammatory bowel disease in daily routine. But the variables, most effective in these analyses, may be helpful for valid discrimination of Crohn's disease and ulcerative colitis in adults. These variables are "deformation of crypt", "stromal fibrosis" and "number of eosinophils".