||Objective: Multivitamin supplements are used by nearly half of middle-aged women in the USA. Despite this high prevalence of multivitamin use, little is known about the effects of multivitamins on health outcomes, including cancer risk. Our main objective was to determine the association between multivitamin use and the risk of breast cancer in women. Design: We conducted a population-based case-control study among 2968 incident breast cancer cases (aged 20-69 years), diagnosed between 2004 and 2007, and 2982 control women from Wisconsin, USA. All participants completed a structured telephone interview which ascertained supplement use prior to diagnosis, demographics and risk factor information. Odds ratios and 95% confidence intervals were calculated using multivariable logistic regression. Results: Compared with never users of multivitamins, the OR for breast cancer was 1.02 (95% CI 0.87, 1.19) for current users and 0.99 (95% CI 0.74, 1.33) for former users. Further, neither duration of use (for >= 10 years: OR = 1.13, 95% CI 0.93, 1.38, P for trend 0.25) nor frequency (> 7 times/week: OR = 1.00, 95% CI 0.77, 1.28, P for trend 0.97) was related to risk in current users. Stratification by menopausal status, family history of breast cancer, age, alcohol, tumour staging and postmenopausal hormone use did not significantly modify the association between multivitamin use and breast cancer. Conclusions: The current study found no association between multivitamin supplement use and breast cancer risk in women.