After the war, Doll returned to St Thomas' to research asthma. In 1948 he joined a research team under Dr Francis Avery-Jones at the Central Middlesex Hospital, run under the auspices of the statistical research unit of the Medical Research Council. Over a 21-year career in the unit, Doll rose to become its director. His research there initially focused on the role of occupational factors in causing peptic ulcers. In 1950, he then undertook with Austin Bradford Hill a study of lung cancer patients in 20 London hospitals, at first under the belief that it was due to the new material tarmac, or motor car fumes, but rapidly discovering that tobacco smoking was the only factor they had in common. Doll himself stopped smoking as a result of his findings, published in the British Medical Journal in 1950, which concluded:
The risk of developing the disease increases in proportion to the amount smoked. It may be 50 times as great among those who smoke 25 or more cigarettes a day as among non-smokers.
Four years later, in 1954 the British doctors study, a study of some 40 thousand doctors over 20 years, confirmed the suggestion, based on which the government issued advice that smoking and lung cancer rates were related.
In 1955 Doll reported a case controlled study that has firmly established the relationship between asbestos and lung cancer.
Doll is distinguished for his researches in epidemiology, and particularly the epidemiology of cancer where in the last 10 years he has played a prominent part in (a) elucidating the causes of lung cancer in industry (asbestos, nickel & coal tar workers) & more generally, in relation to cigarette smoking, and (b) in the investigation of leukaemia particularly in relation to radiation, where using the mortality of patients treated with radiotherapy he has reached a quantitative estimate of the leukaemogenic effects of such radiation. In clinical medicine he has made carefully controlled trials of treatments for gastric ulcer. He has been awarded the United Nations prize for outstanding research into the causes & control of cancer & the Bisset Hawkins medal of the Royal College of Physicians for his contributions to preventative medicine.
Initially, epidemiology was held in low regard, but in his time at Oxford he helped reverse this. He was the primary agent behind the creation of Green College, which was founded in 1979. Doll was appointed the first Warden of Green College, from where he retired in 1983. Green College merged with Templeton College in 2008 to become Green Templeton College, which is located on the site that was previously Green College.