Studies of the morphologic and phenotypic diversity of intraepithelial T cells in human small intestine have shown them to be heterogeneous, yet distinct from most extra intestinal T cells. In this study sequential immunoenzymatic staining was used to define new intraepithelial lymphocyte subpopulations in man. In normal human jejunum approximately 6% of the intraepithelial T cells expressing CD3 (an antigen associated with the T cell receptor) do not express the T cell subset antigens CD4 or CD8. Approximately 20% of CD7+ cells (T cells and null cells) do not express CD4 or CD8 and 14% of the CD7+ cells do not express CD3 and are therefore not T cells. The CD7+, CD3+/-, CD4-, CD8- population is concentrated in the tips of the villi. In coeliac disease, the ratios of the subsets change significantly. The percentage of CD3+, 4-, 8- cells increases to 28%, the proportion of CD7+, 4-, 8- cells remains unchanged and the CD7+, CD3- (non-T cell) population is reduced to 1.4% of the CD7+ cells. In contrast, in patients with villous atrophy of uncertain aetiology, all CD4-, CD8- lymphocyte subsets are decreased compared with normal biopsies. Finally, in enteropathy associated T cell lymphoma (malignant histiocytosis of the intestine) in which the 'uninvolved mucosa' is histologically similar to untreated coeliac disease, the changes in the intraepithelial T cell sub-sets are indistinguishable from those in coeliac disease, suggesting that the lymphoma is a complication of coeliac disease.