For one thing, the new conflicts with other, much larger studies. In a study published in 1997, Kaiser-Permanente researchers followed 65,000 patients for 10 years and saw no sign of marijuana use increasing the risk of lung cancer or other smoking-related cancers. And a UCLA study similar in design to this one, published in 2006, found a trend toward lower lung cancer rates among marijuana smokers. Instead of 79 cancer cases, the UCLA team looked at 1,212. The result was so striking that they speculated that it "may reflect a protective effect of marijuana."

http://www.alternet.org/story/76496/