We examined recalled measures of adolescent diet, physical activity, and body size in relation to adult breast density in 201 U.S. Chinese immigrant women recruited in January 2002 to May 2003 from Philadelphia region screening programs. Mammographic images were classified into 1 of 4 categories ranging from "entirely fatty" to "extremely dense." Questionnaires assessed diet and physical activity between ages 12-17, relative weight and height at age 10, and weight at age 18. To estimate odds ratios (ORs), we conducted logistic regression analyses using proportional odds models for polychotomous outcomes. Higher adult breast density was significantly associated with adolescent red meat intake (adjusted 3rd vs. 1st tertile OR = 3.0, 95% confidence interval (CI) 1.5-6.4, trend P = 0.003) but not with other adolescent factors. For the association of adult acculturation with breast density, adjustment for adolescent red meat intake attenuated the OR for the highest vs. lowest level of acculturation from 2.5 (95% CI 1.2-5.3) to 1.9 (95% CI 0.9-4.0). Greater adolescent red meat intake may have increased adult breast density and partly accounted for the strong association between acculturation and breast density in this sample of immigrant Chinese women. If confirmed by further study, dietary prevention efforts for breast cancer should be considered earlier in life.