On average, cigarette smokers die ten years younger than non-smokers, according to a 50-year-long study of smoking and death among British doctors, published in this week's British Medical Journal*. But stopping at age 50 halves the risk, and stopping at 30 avoids almost all of it.

Sir Richard Doll, emeritus Professor of Medicine at the Clinical Trial Service Unit (CTSU), University of Oxford, launched the study in 1951 when he himself was in his 30s. Now in his 90s, he reveals the final results. Among the doctors born between 1900 and 1930, about half of the cigarette smokers were killed by their habit. However, there is a unique group, born around 1920, among whom two-thirds of those who continue to smoke cigarettes are killed.

The study, funded by the Medical Research Council, Cancer Research UK and the British Heart Foundation, also demonstrates the benefits of quitting smoking at any age. Stopping smoking at ages 60, 50, 40 or 30 gains, respectively, about 3, 6, 9 or 10 years of life expectancy.

The initial results were published by Richard Doll in the BMJ on June 26, 1954, confirming that smoking tobacco caused lung cancer. The 50-year results are being published by him on the exact 50th anniversary (June 26, 2004) of those initial results, showing that the overall risks are much greater than originally suspected.

British men born in the first few decades of the 20th century are the first population in the world in which the full hazards of long-term cigarette smoking, and the corresponding benefits of stopping, can be assessed directly.

Doctors were chosen to be the subjects of the study because they were a relatively simple group to follow through the Medical Register held by the General Medical Council.

Sir Richard Peto, Professor of Medical Statistics and Epidemiology at Oxford, who has collaborated on the study for 30 years, says: "On average, those who continue to smoke lose 10 years of life but stopping smoking at ages 60, 50, 40 or 30 gains 3, 6, 9 or the full 10 years of life expectancy. Of those who continued to smoke, half were killed by their habit."