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Background Carotenoids, micronutrients in fruits and vegetables, may reduce breast cancer risk. Most, but not all, past studies of circulating carotenoids and breast cancer have found an inverse association with at least one carotenoid, although the specific carotenoid has varied across studies. Methods We conducted a pooled analysis of eight cohort studies comprising more than 80% of the world's published prospective data on plasma or serum carotenoids and breast cancer, including 3055 case subjects and 3956 matched control subjects. To account for laboratory differences and examine population differences across studies, we recalibrated participant carotenoid levels to a common standard by reassaying 20 plasma or serum samples from each cohort together at the same laboratory. Using conditional logistic regression, adjusting for several breast cancer risk factors, we calculated relative risks (RRs) and 95% confidence intervals (CIs) using quintiles defined among the control subjects from all studies. All Pvalues are two-sided. Results Statistically significant inverse associations with breast cancer were observed for (β-carotene (top vs bottom quintile RR = 0.87, 95% CI = 0.71 to 1.05, Ptrend=.04), [β-carotene (RR = 0.83, 95% CI = 0.70 to 0.98, Ptrend=.02), lutein+zeaxanthin (RR = 0.84, 95% CI = 0.70 to 1.01, Ptrend=.05), lycopene (RR = 0.78, 95% Cl = 0.62 to 0.99, Ptrend=.02), and total carotenoids (RR = 0.81, 95% CI = 0.68 to 0.96, Ptrend=.01). [β-Cryptoxanthin was not statistically significantly associated with risk.Tests for heterogeneity across studies were not statistically significant. For several carotenoids, associations appeared stronger for estrogen receptor negative (ER-:) than for ER tumors (eg, [β-carotene: ER-: top vs bottom quintile RR = 0.52, 95% CI = 0.36 to 0.77, Ptrend=.001; ER+: RR = 0.83, 95% CI = 0.66 to 1.04, Ptrend=.06; Pheterogeneity=.01 ), Conclusions This comprehensive prospective analysis suggests women with higher circulating levels of α-carotene, [β-carotene, lutein+zeaxanthin, lycopene, and total carotenoids may be at reduced risk of breast cancer. [ABSTRACT FROM AUTHOR]