The 'epidemiological necropsy' is a newly proposed research strategy in which the size and composition of the epidemiological reservoir of undetected disease is estimated from the relative frequency of necropsy surprise patients, in whom the disease was not suspected during life. The current study was done to help validate a basic premise of the strategy. We examined the surprise necropsy discovery of two upper gastrointestinal malignancies: oesophageal cancer, for which an undetected reservoir would not be expected because the cancer has little room to grow, and gastric cancer, for which a sizeable reservoir might be anticipated. In a review of 15,812 necropsies during 1953-1982 at Yale-New Haven Hospital, 70 cases of oesophageal cancer were identified postmortem. Except for five surprise cases, located at the gastro-oesophageal junction where there is room to grow, no oesophageal cancer reservoir was found during the 30-year period. In the same secular period, however, a distinctive set of reservoir cases was found among 162 necropsy instances of gastric cancer. About two-thirds of these gastric cancers had been previously diagnosed during life, but the remainder consisted of either necropsy surprise cases or patients with wrong primary cancer diagnoses during life. The secular rates of occurrence remained stable and similar in both sexes for surprise gastric cancer cases during the three 10-year periods from 1953-1982, but no wrong primary diagnoses occurred during 1978-1982, after the introduction of improved methods of premortem diagnosis. The results help validate the cancer reservoir theory for malignancies that have room to grow, and confirm the concept that the 'epidemiological necropsy' can reflect qualitative and quantitative changes in cancer reservoirs.