A study in the Journal of the American Medical Association (Dec 1996; 276: 1968 - 1973), based on data gathered in the Seattle area from 1992-1994, found that helmets reduced "the risk of any head injury by 69%, brain injury by 65%, and severe brain injury by 74%."

In another study in the journal Pediatrics (Vol. 122 No. 3 September 2008, pp. 605-610), a bicycle helmet law enacted in Ontario was shown to reduce the number of annual deaths of bicycle riders under 15 by 52%, while the rate of riders wearing helmets in the age category went from 45% pre-law to 65% post-law. The authors found that 89% of fatalities in the province over the studied time period were directly related to head injury.

The most important statistic to remember is that it only takes one time to bust your head open if you crash while riding your bicycle and that one hit can be deadly. I have wore a helmet on my bicycle for years. Two years ago, I can crashed and hit the pavement hard with my head. My helmet was destroyed and I had a headache but I was alive. If I was not wearing a helmet I would either be dead or severely brain traumatized. Always wear a helmet.

I am editing this answer because of something that just happened to my son who is a road cyclist. He got hit be someone who ran a red light and then ran from the scene. He was thrown across the intersection and hit his head hard on the pavement. He says he would be dead if he had not been wearing a helmet. Sure, he will have to replace the helmet now but he walked away with minor injuries.

Wear a helmet. PERIOD.