A survey has found that women smokers can earn themselves ten years of extra life by quitting before middle age.
The study of 1.3 million women found that smoking tripled the chances of dying over nine years compared with non-smokers.
Most of the increased death rate resulted from smoking-related diseases such as lung cancer, chronic lung disease, heart disease or stroke.
The risk rose steeply with the quantity of tobacco smoked, but even light smokers doubled their likelihood of dying.
Smokers who kicked the habit around age 30 avoided 97% of their excess risk of premature death.
The authors of the Million Women Study wrote in The Lancet medical journal: "Smokers lose at least ten years of lifespan.
"Although the hazards of smoking until age 40 years and then stopping are substantial, the hazards of continuing are ten times greater."
Women aged 50 to 65 were enrolled into the study, designed to investigate links between health and lifestyle, from 1996 to 2001.
Participants completed a questionnaire about living habits, medical and social factors and were re-surveyed three years later.
Women were monitored for a total of 12 years on average, during which there were 66,000 deaths.
Initially, 20% of the women were smokers, 28% were ex-smokers, and 52% had never smoked.
Those who still smoked at the three-year re-survey were almost three times more likely than non-smokers to die over the next nine years.
Both the hazards of smoking and the benefits of quitting were greater than previous studies had suggested, said the researchers.
Co-author Professor Richard Peto, from Oxford University, said: "If women smoke like men, they die like men, but, whether they are men or women, smokers who stop before reaching middle age will on average gain about an extra ten years of life.
"Both in the UK and in the USA, women born around 1940 were the first generation in which many smoked substantial numbers of cigarettes throughout adult life.
"Hence, only in the 21st Century could we observe directly the full effects of prolonged smoking, and of prolonged cessation, on premature mortality among women."