A series of 267 gastroenteropancreatic endocrine tumours has been revised from the point of view of histopathologic diagnosis, hormonal profile and clinical behaviour. Results of this investigation, together with revised concepts on the histogenesis of gastroenteropancreatic endocrine growths, allowed to develop detailed classification systems which proved useful for precise tumour diagnosis and for clinicopathologic correlation, with special reference to tumour function, prognosis and therapy. Among 132 pancreatic growths, various types of islet cell tumours (61 cases), with (45 cases) or without (16 cases) hyperfunctional syndrome, were separated from different types of gut-related (38 cases) and 'ectopic' (three cases) tumours, as well as from 25 non-functioning, locally symptomatic tumours, three small cell carcinomas and two mixed endocrine-exocrine tumours. Among 97 intestinal tumours, 39 argentaffin EC cell carcinoids, mostly from the appendix and ileum, were separated from 23 hindgut-type carcinoids, mostly from the rectum, 22 gastrin cell tumours, mainly from the duodenal bulb, five somatostatin cell tumours, mostly from the periampullary region of the duodenum, and two gangliocytic paragangliomas. Among 38 gastric tumours, five small cell 'neuroendocrine' carcinomas were separated from three gastrin cell tumours and 30 argyrophil carcinoids, 27 of which arose in the body fundus, 16 associated with chronic atrophic gastritis and four with combined Zollinger Ellison/Multiple Endocrine Neoplasia Syndrome.