ARTICLE ABSTRACTBackground: Mitochondria play an important role in cellular energy metabolism, free radical production, and apoptosis, and thus may be involved in cancer development.Methods: We evaluated mitochondrial DNA (mtDNA) copy number in peripheral leukocytes in relation to colorectal cancer risk in a case–control study of 444 colorectal cancer cases and 1,423 controls nested in the Shanghai Women's Health Study, a population-based, prospective cohort study. Relative mtDNA copy number was determined by a quantitative real-time PCR assay using peripheral leukocyte DNA samples collected at the time of study enrollment, before cancer diagnosis.Results: We found that baseline mtDNA copy number was lower among women who subsequently developed colorectal cancer [geometric mean, 0.277; 95% confidence interval (CI), 0.269–0.285] than among women who remained cancer-free (geometric mean, 0.288; 95% CI, 0.284–0.293; P = 0.0153). Multivariate adjusted ORs were 1.26 (95% CI, 0.93–1.70) and 1.44 (95% CI, 1.06–1.94) for the middle and lower tertiles of mtDNA copy number, respectively, compared with the upper tertile (highest mtDNA copy number; Ptrend = 0.0204). The association varied little by the interval between blood collection and cancer diagnosis.Conclusions: Our data suggest that mtDNA copy number measured in peripheral leukocytes may be a potential biomarker useful for colorectal cancer risk assessment.Impact: If confirmed, mtDNA copy number measured in peripheral leukocytes may be a biomarker useful for colorectal cancer risk assessment. Cancer Epidemiol Biomarkers Prev; 23(11); 2357–65. ©2014 AACR.