During the last decades we have seen dramatic changes worldwide in the epidemiology of gastrointestinal cancers, in particular the incidence of proximal adenocarcinomas. Oesophageal adenocarcinoma, once a rare histological type, has in most Western countries outnumbered squamous cell carcinoma and has now become the most common tumour in the oesophagus. Several reports assert oesophageal adenocarcinoma to currently having the fastest increasing rates among all tumours. For adenocarcinoma of the stomach, in particular the more distal parts of the stomach, the incidence rate has gradually declined moving from being the most common tumour type globally in the early 20^th century to being the number 4 cancer today. Colorectal adenocarcinoma incidence rates have in most countries been reported as a steady but slow rise during the same time period.

These incidence alterations are in large yet unexplained. Major incidence variations, during the span of decades only, should not be attributable to genetic alterations but by changes in exposure to environmental factors such as life style habits, infections, dietary factors, irradiation, or air-borne pollutants. Such associations can seldom, if ever, be studied by the ‘gold-standard’ of clinical research – the randomized controlled trial. Carefully designed and executed epidemiological, or observational, research is a robust and strong tool to approach such questions and can provide firm evidence of causality, however.

As an example I would like to share some of the results and experience of the observational research that has been undertaken the last decade due to the concerns of the unexplained rapid rise of oesophageal adenocarcinoma incidence.