In 1922, Hill went to work for the Industry Fatigue Research Board. He was associated with the medical statisticianMajor Greenwood and, to improve his statistical knowledge, Hill attended lectures by Karl Pearson. When Greenwood accepted a chair at the newly formed London School of Hygiene and Tropical Medicine, Hill moved with him, becoming Reader in Epidemiology and Vital Statistics in 1933 and Professor of Medical Statistics in 1947. Hill had a distinguished career in research and teaching and as author of a very successful textbook, Principles of Medical Statistics, but he is famous for two landmark studies. He was the statistician on the Medical Research Council Streptomycin in Tuberculosis Trials Committee and their study evaluating the use of streptomycin in treating tuberculosis, is generally accepted as the first randomised clinical trial. The use of randomisation in agricultural experiments had been pioneered by Ronald Aylmer Fisher. The second study was rather a series of studies with Richard Doll on smoking and lung cancer. The first paper, published in 1950, was a case-control study comparing lung cancer patients with matched controls. Doll and Hill also started a long-term prospective study of smoking and health. This was an investigation of the smoking habits and health of 40,701 British doctors for several years. Fisher was in profound disagreement with the conclusions and procedures of the smoking/cancer work and from 1957 he criticised the work in the press and in academic publications. In 1965, built upon the work of Hume and Popper, Hill suggested several aspects of causality in medicine and biology, which have remained in use by epidemiologists to date. On Hill's death in 1991, Peter Armitage wrote, "to anyone involved in medical statistics, epidemiology or public health, Bradford Hill was quite simply the world's leading medical statistician."
Has, by the application of statistical methods, made valuable contributions to our knowledge of the incidence and aetiology of industrial diseases, of the effects of internal migration upon mortality rates, and of the natural and experimental epidemiology of various infections, for example of the risks of an attack of poliomyelitis following inoculation procedures and of the risk of congenital abnormalities being precipitated by maternal rubella in the pregnant woman. Since the war he has demonstrated in an exact and controlled field survey the association between cigarette smoking and the incidence of cancer of the lung, and has been the leader in the development in medicine of the precise experimental methods now used nationally and internationally in the evaluation of new therapeutic and prophylactic agents.