Morphometric techniques for quantitating cytologic and volumetric changes in small intestinal mucosae are described: measurements were controlled with respect to a constant 'test area' of muscularis mucosae, thus following valid comparisons between normal and damaged mucosae. Procedures for enumerating cell populations within mucosal volume compartments are illustrated by analyzing lymphocytes within surface epithelium. These techniques serve to distinguish (i) absolute cell counts from (ii) relative cell counts ("densities") the latter being shown to be considerably affected by changes in epithelial volume. They also permit construction of mathematical models e.g. cubes of equivalent volume, volume-density graphs, proportional volumes, which are illustrated. Use of these morphometric procedures showed that there is no major difference in epithelial lymphocyte populations between untreated coeliac disease, and control, mucosae. The data thus fail to support the widespread view that "infiltration" of coeliac disease epithelium by lymphocytes represents a local cell-mediated immune reaction to gluten.