There has been recent controversy as to whether green tea can prevent or fight cancer. There are studies that support this claim and others that dismiss it.
Current Status
After review of studies concerning green tea and cancer, the FDA found that the consumption of green tea had any effect on reducing the risk factor for cancer.

From the FDA:

"Under the Food and Drug Administration's (FDA) "Consumer Health for Better Nutrition Initiative," the Agency is announcing the results of a review of qualified health claims that green tea may reduce the risk of certain types of cancer. Based on a systematic evaluation of the available scientific data, the FDA intends to consider exercising its enforcement discretion for the following qualified health claims for breast and prostate cancer:

"Two studies do not show that drinking green tea reduces the risk of breast cancer in women, but one weaker, more limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of breast cancer"; and

"One weak and limited study does not show that drinking green tea reduces the risk of prostate cancer, but another weak and limited study suggests that drinking green tea may reduce this risk. Based on these studies, FDA concludes that it is highly unlikely that green tea reduces the risk of prostate cancer."

The FDA also concluded that existing evidence does not support qualified health claims for green tea consumption and a reduced risk of any other type of cancer.

Guidance on qualified health claims for conventional foods and dietary supplements was issued by the FDA in July 2003. FDA will continue to evaluate new information that becomes available to determine whether changes in these claims, or in the decision, are necessary."
What's at Stake?
Many researchers feel that if green tea did prevent cancer, then it would have to be consumed in large qualtities. According to the Mayo Clinic, certain animal studies suggest that consuming large amounts of green tea may be carcinogenic.