Neuroendocrine tumours of the gastroenteropancreatic tract are growths originating either from the cells of the diffuse (neuro)endocrine system, such as gastric carcinoids and islet cell tumours, or from nerve structures, such as duodenal paragangliomas. A great deal of cellular and clinical information is available whereas data concerning the genetic and molecular basis of diffuse (neuro)endocrine system tumours of the gastroenteropancreatic tract are very few and fragmentary. The present paper reviews some genetic and molecular investigations of potential interest. As far as concerns the genetic background of diffuse (neuro)endocrine system tumours, the frequent loss of heterozygosity for the locus of Multiple Endocrine Neoplasia type 1 in tumour samples suggests a potential role of the Multiple Endocrine Neoplasia gene. With regard to the molecular background, no mutation of the p53 or retinoblastoma susceptibility (Rb) genes has been demonstrated. Useful data have been generated by in situ analysis of the proliferation activity of tumours.