AbstractThere are racial/ethnic differences in the incidence of hormone receptor positive and negative breast cancer. To understand why these differences exist, we investigated associations between hormone‐related factors and breast cancer risk by race/ethnicity in the Multiethnic Cohort (MEC) Study. Among 81 511 MEC participants (Native Hawaiian, Japanese American, Latina, African American and White women), 3806 estrogen receptor positive (ER+) and 828 ER− incident invasive breast cancers were diagnosed during a median of 21 years of follow‐up. We used Cox proportional hazards regression models to calculate associations between race/ethnicity and breast cancer risk, and associations between hormone‐related factors and breast cancer risk by race/ethnicity. Relative to White women, ER+ breast cancer risk was higher in Native Hawaiians and lower in Latinas and African Americans; ER− disease risk was higher in African Americans. We observed interaction with race/ethnicity in associations between oral contraceptive use (OC; P_int .03) and body mass index (BMI; P_int .05) with ER+ disease risk; ever versus never OC use increased risk only in Latinas and positive associations for obese versus lean BMI were strongest in Japanese Americans. For ER‐ disease risk, associations for OC use, particularly duration of use, were strongest for African Americans (P_int .04). Our study shows that associations of OC use and obesity with ER+ and ER− breast cancer risk differ by race/ethnicity, but established risk factors do not fully explain racial/ethnic differences in risk. Further studies are needed to identify factors to explain observed racial/ethnic differences in breast cancer incidence.