This study was undertaken to assess the prevalence and characteristic hormonal profile of endocrine cells in Barrett's mucosa and to determine to what extent this profile was shared by endocrine cells of adenocarcinomas arising therefrom. In addition, lower oesophageal carcinomas, not associated with columnar metaplasia, were examined to see if they exhibited a different hormonal profile. The patients studied comprised 43 who had had multiple oesophageal biopsies. 35 who had had oesophagogastric resection for adenocarcinoma arising in Barrett's mucosa and 26 in whom the resection showed no metaplastic epithelium adjacent to tumour. Argyrophil cells were present in 90% of biopsies and resections of Barrett's mucosa combined, irrespective of the histological type of metaplastic epithelium. By immunocytochemistry the most frequently identified substance in mucosal endocrine cells was serotonin (82%) followed by somatostatin (54%), secretin (22%) and pancreatic polypeptide (17%). Gastrin, bombesin, cholecystokinin, ACTH and substance P were not identified in metaplastic mucosa in any case. The difference in expression of serotonin by endocrine cells of tumours arising in Barrett's mucosa (31%) and those not (3.8%) was statistically significant (P less than 0.0186). Carcinoembryonic antigen (CEA) was demonstrated in 60% of oesophageal carcinomas, both endocrine positive and endocrine negative. Focal CEA expression was seen in 4.6% of biopsies and 14% of Barrett's mucosa adjacent to tumour. These results indicate a higher prevalence of endocrine cells in Barrett's mucosa than hitherto documented and suggest that serotonin may be a useful marker in distinguishing between primary oesophageal and putative gastric cancers at the gastro-oesophageal junction. The identification of CEA in oesophageal columnar epithelium is of little value in predicting the development of malignancy.