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Hair products can contain hormonally active and carcinogenic compounds. Adolescence may be a period of enhanced susceptibility of the breast tissue to exposure to chemicals. We therefore evaluated associations between adolescent hair product use and breast cancer risk. Sister Study participants (ages 35‐74 years) who had completed enrollment questionnaires (2003‐2009) on use of hair dyes, straighteners/relaxers and perms at ages 10 to 13 years (N = 47 522) were included. Cox proportional hazards regression was used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (95% CIs) for associations between hair products and incident breast cancer (invasive cancer or ductal carcinoma in situ), with consideration of heterogeneity by menopausal status and race/ethnicity. Over an average of 10 years of follow‐up, 3380 cases were diagnosed. Frequent use of straighteners and perms was associated with a higher risk of premenopausal (HR = 2.11, 95% CI: 1.26‐3.55 and HR = 1.55, 95% CI: 0.96‐2.53, respectively) but not postmenopausal breast cancer (HR = 0.99, 95% CI: 0.76‐1.30 and HR = 1.09, 95% CI: 0.89‐1.35, respectively). Permanent hair dye use during adolescence was uncommon (<3%) and not associated with breast cancer overall (HR = 0.97, 95% CI: 0.78‐1.20), though any permanent dye use was associated with a higher risk among black women (HR = 1.77, 95% CI: 1.01‐3.11). Although frequency of use of perms (37% non‐Hispanic white vs 9% black) and straighteners (3% non‐Hispanic white vs 75% black) varied by race/ethnicity, associations with breast cancer did not. Use of hair products, specifically perms and straighteners, during adolescence may be associated with a higher risk of premenopausal breast cancer.

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