The prostate cancer study by the Journal of the National Cancer Institute was the largest of its kind and involved 48,000 men. Those who drank one to three cups a day had a 30 percent lower risk of developing lethal prostate cancer tumors; the men who drank drinking six or more cups a day had a 60 percent lower risk.
For the women in the Karolinksa Institutet study, drinking five cups of coffee a day was linked to a 20% decrease in overall breast cancer risk, after adjusting for participants' ages. There was a much stronger reduction in risk—57%—for a very aggressive type of breast cancer called estrogen-receptive negative cancers.
I've somewhat guiltily downed increasing amounts of caffeine as my medical education and then employment has gone on. Seems it may not kill me any faster.