 When Keen returned to London from the United States in 1961, he was hired as a lecturer by Guy's Hospital and its associated medical school, where he would spend the rest of his career.In 1962, he conducted the Bedford Survey, in which every adult in Bedford was asked to provide a urine sample in order to study the population prevalence of diabetes; around 70% of the population provided samples, and 250 participants were found to have undiagnosed diabetes as a result.The study led to the first definition of prediabetes, which Keen called "borderline diabetes", and demonstrated the relationship between glucose intolerance and cardiovascular disease at a population level.