We investigated the relationship between methylxanthine consumption and breast cancer using data from a case-control study which included 1,510 cases and 1,882 controls identified through a nation-wide breast cancer screening program. There was no evidence of a positive association between methylxanthine consumption and risk of breast cancer. In fact, there was some suggestion of a negative association, particularly in women diagnosed after age 50. In addition, there was no evidence of increased risk with past or recent methylxanthine consumption, or with the consumption of caffeine or specific beverages, most notably brewed or instant caffeinated coffee and tea.