Enteropathy associated T cell lymphoma (EATCL) is the most serious complication of coeliac disease. HLA genotyping shows that patients with EATCL have the coeliac disease associated DQA1*0501, DQB1*0201 phenotype. Other HLA-DR/DQ alleles that may be associated with adult onset coeliac disease appear to represent additional risk factors for lymphoma development. Increased numbers of small intestinal intraepithelial cytotoxic T cells are found in the small intestinal mucosa of patients with coeliac disease and in the enteropathic bowel of patients with EATCL. The neoplastic cells of EATCL have the immunophenotype of intraepithelial cytotoxic T cells and may exhibit epitheliotropism. Analysis of T cell receptor genes and immunohistochemistry have shown that the intestinal mucosa distant from the tumour contains clonal populations of small T cells, often of the same clone as the high grade T cell lymphoma. Most cases of chronic ulcerative enteritis are probably part of the same disease process. The ulceration seen in chronic ulcerative enteritis and in enteropathy associated T cell lymphoma may be due to the release of cytolytic enzymes by cytotoxic T cells and tumour cells. These findings suggest that EATCL arises in the setting of coeliac disease and evolves from intraepithelial lymphocytosis through low grade lymphoma to a high grade tumour, possibly under antigen drive from gliadin peptides. These peptides may be presented to the intraepithelial cytotoxic T cells directly by epithelial cells bearing the coeliac disease associated HLA-DQ alleles.