This paper assesses the extent and importance of geographical variation in disease presentation in relation to automated systems for clinical decision making. Reference is made to studies carried out by the World Organization of Gastroenterology involving 1,500 patients with inflammatory bowel disease, 6,000 patients with acute abdominal pain, and 1,300 patients with upper GI bleeding. These studies indicate that geographical variation in disease presentation (both regarding prior probability of disease and conditional probability of symptoms in each disease) is so great as to constitute a considerable potential threat to the widespread introduction of any automated decision-making system. It is suggested that there is an urgent need for the creation of large-scale multinational case series where diseases and symptoms are adequately and reproducibly defined in advance. Reference to studies previously mentioned is used to adduce evidence of the benefits of such large-scale data bases.