In 1951, 34,439 male British doctors joined a study on the health effects of smoking. In June 2004, researchers published a 50-year follow-up on the original docs, 6,000 of whom were still alive and in the study.

What the researchers wanted to know: So, really: How bad are cigarettes?

What they did: Every few years, the doctors filled out a questionnaire on their smoking habits. The researchers kept tabs on doctors' deaths. Cigarettes didn't become popular until the early 20th century, so men born later were more likely to have started smoking lots of cigarettes young.

What they found: Of the persistent smokers—men who didn't quit—who were born between 1900 and 1930, about half were killed by their habit. Mortality rates were even higher among men born in the 1920s. The persistent smokers lost an average of 10 years of life, compared to nonsmokers. Some, the researchers say, lost decades of life to smoking. Men who stopped smoking at 30 gained about 10 years of life. Oh, and two men died in fires because they were smoking in bed.

What the study means to you: Yes, it seems kind of silly to have yet another study showing that smoking is bad for you, but back in the 1950's, this was one of the first studies to show it, and it's incredibly useful to have this solid, long-term data.

Caveats: Cigarettes may be safer now. Many of the smokers also drank, and it's impossible to separate the effects of the two in this study.

Find out more: Read the Surgeon General's 1964 report that raised the alarm on smoking.

Read the article: Doll, R., Peto, R., Boreham, J., and I. Sutherland. "Mortality in Relation to Smoking: 50 Years' Observations on Male British Doctors." British Medical Journal. June 26, 2004, Vol. 328, p. 1519.

Article online: bmj.bmjjournals.com