This study is based on the clinical records and post-mortem findings of 264 patients with leukaemia. Gross leukaemic lesions in the gastrointestinal tract were found in 39 patients, an overall incidence of 14·8%. The incidence in all types of acute leukaemia was 18·4%, in chronic leukaemias 9·6%, and in myeloid leukaemia 10·9%. The ileum, stomach, and proximal colon were the sites most commonly affected. Four types of lesion were found: raised leukaemic nodules, leukaemic plaques, diffuse infiltrations with a convoluted brain-like appearance of the mucosal folds, and a multiple leukaemic polyposis. The clinical and pathological features of these lesions are described, and references are given to similar cases reported in the literature.