The association of caffeine intake and postmenopausal breast cancer incidence was assessed among 34,388 Iowa women aged 55-69 years in 1986 and followed through 1990. Caffeine intake was assessed by a food frequency questionnaire. Median caffeine intake was 212 mg/day in women who developed breast cancer (n = 580) and 201 mg/day in women who remained free of the disease (p = 0.95). There was no apparent association between breast cancer occurrence and quintile of caffeine intake, either adjusted for age or for multiple breast cancer risk factors. The same was true for regular coffee and other caffeine-containing foods.