The researchers concluded, "These data show no overall association between caffeine consumption and breast cancer risk. The possibility of increased risk in women with benign breast disease or for tumors that are estrogen and progesterone-receptor negative or larger than 2 cm warrants further study."

Reference: Ishitani K, Lin J, Manson J, et al. Caffeine Consumption and the Risk of Breast Cancer in a Large Prospective Cohort of Women. Archives of Internal Medicine. 2008; 168: 2022-2031.