A new joint study out of Harvard and Tokyo indicates that caffeine consumption may not be a strict carcinogen. It found no statistically reliable evidence that drinking coffee increases overall breast cancer risk, but it did find data regarding how it effects pre-existing breast tissue conditions, requiring further study.
The researchers reviewed the medical and dietary records of nearly 40,000 women since a baseline taken between 1992 - 1995, and examined the commonalities between their dietary intake and the development of breast cancer. They were unable to find any statistical proof that caffeine did in fact increase a woman's risk of breast cancer, but they did notice that those with higher caffeine intake (more than 4 cups per day) did experience adverse effects in the state of pre-existing benign breast disease and tumors.
While it appears that minimal intake my not increase your risk -- if you're a coffee lover with a history of breast cancer in your family, it might be a good idea to switch to decaf (just to be on the safe side!).