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BACKGROUND: Uterine fibroids disproportionately affect Black women, and exposure to chemicals from hair relaxers or straighteners ('straighteners') may contribute to fibroid development. OBJECTIVES: We examined the association between straightener use and prevalent young-onset uterine fibroids (diagnosed before age 36 y), as well as incident fibroids (diagnosed age 36-60 y), with a focus on Black women. We also examined differences in associations across birth cohorts as proxies for formulation changes. METHODS: Data from 4,162 Black women in the Sister Study, a prospective cohort of women 35-74 y of age (enrolled 2003-2009), were analyzed. We used logistic regression to estimate odds ratios (ORs) for the association of straightener use at 10-13 y of age and self-reported young-onset fibroids. We used Cox regression to assess hazard ratios (HRs) for straightener use (age 10-13 y and in 12 months before enrollment) and incident fibroids among 779 premenopausal Black women. Similar analyses were conducted in 40,782 non-Hispanic White women. RESULTS: Over 70% of Black women used straighteners. In comparison with no use, any [OR= 1:15; 95% confidence interval (CI): 0.96, 1.36] and frequent use (OR = 1:18; 95% CI: 0.99, 1.42) of straighteners at 10-13 y of age were associated with young-onset fibroids. This association was most apparent among those born between 1928 and 1945 (OR = 1:78; 95% CI: 1.15, 2.74) and 1965-1974 (OR = 1:64; 95% CI: 0.82, 3.29). Fibroid incidence from 36 to 60 y of age was modestly associated with use of straighteners at 10-13 y of age [hazard ratio (HR) = 1:14; 95% CI: 0.81, 1.63] and in the 12 months prior to enrollment (HR= 1:32; 95% CI: 0.88, 2.00). Among non-Hispanic White women, straightener use at 10-13 y of age was similarly associated with young-onset fibroids (OR = 1:23; 95% CI: 1.03, 1.47) despite lower use ( DISCUSSION: Hair straightener use may be positively associated with fibroid risk. https://doi.org/10.1289/EHP14493, Introduction Uterine fibroids (leiomyomas) are common hormone-dependent benign pelvic tumors (1) that develop during a woman's reproductive years (2) and are a significant cause of morbidity in the United States. [...]