Carotenoids have antioxidant and antiproliferative properties and may reduce the risk of breast cancer. We examined the association between dietary carotenoids and risk of invasive breast cancer in the Swedish Mammography Cohort, a population-based cohort of 36,664 women who completed a questionnaire in 1997. During a mean follow-up of 9.4 years, 1008 women were diagnosed with incident breast cancer. Dietary carotenoids were not significantly associated with the risk of breast cancer overall or with any subtype defined by oestrogen receptor (ER) and progesterone receptor (PR) status. However, dietary alpha-carotene and beta-carotene were inversely associated with the risk of ER-PR-breast cancer among ever smokers. Among ever smokers, the multivariable relative risks of ER-PR-breast cancer comparing the highest with the lowest quintile of intake were 0.32 (95% confidence interval (CI): 0.11-0.94; P(trend)=0.01) for alpha-carotene and 0.35 (95% CI: 0.12-0.99; P(trend)=0.03) for beta-carotene. The risk of breast cancer also decreased with increasing intakes of alpha-carotene (P(trend) = 0.02) and beta-carotene (P(trend)=0.01) among women who did not use dietary supplements. These findings suggest that dietary alpha-carotene and beta-carotene are inversely associated with the risk of breast cancer among smokers and among women who do not use dietary supplements.