This report of a workshop examines the mechanisms by which tumours of the forestomach and tumours of gastric neuroendocrine cell origin occur in rodents, and considers the predictive value of these tumours for the identification of carcinogenic hazards to humans. Etiological risk factors in rodents are summarized and evidence for the possible carcinogenic mechanisms involved is discussed. Recommendations are presented on how mechanistic data could be used in overall evaluations of carcinogenicity to humans. Summary reports at the beginning of each section use the background scientific reviews and case studies that were prepared before the workshop by individual participants. These individually authored papers formed the basis for the discussions that were held and are also included in this report.