The role of acculturation in the breast cancer risk increase among U.S. Chinese women is unclear. We examined the association between acculturation and breast density in a sample of foreign-born, U.S. Chinese women and examined factors that may explain such an association. Between January 2002 and May 2003, 212 Chinese women were recruited from Philadelphia region screening programs. Cranial-caudal mammographic images were classified into one of four categories ranging from "entirely fatty" to "extremely dense." Questionnaires assessed information on sociodemographic, cultural, reproductive, and lifestyle factors. An index of acculturation was created based on self-reported English proficiency and within- and cross-ethnicity social interactions. To estimate odds ratios (OR) for falling into a higher versus lower category for breast density, we conducted logistic regression analysis using proportional odds models for polychotomous outcomes. We found that women in the highest acculturation category had denser breasts [age-adjusted OR, 3.1; 95% confidence interval (95% CI), 1.6-6.0]. They also had fewer live births, higher age at first live birth, and higher dairy food intake, all factors associated with breast density. In 196 women with complete covariate data, only adjustment for number of live births and dairy food intake attenuated the estimate for acculturation by >10%. With adjustment for both simultaneously, the most acculturated women were still more likely to have denser breasts (age- and menopause-adjusted OR, 2.0; 95% CI, 1.0-4.2). These analyses are the first to show breast density differences by level of acculturation among foreign-born, U.S. Chinese women. Despite reproductive and lifestyle differences by level of acculturation, differences in these factors did not explain the acculturation-breast density association. Future longitudinal research will examine whether the association is due to early-life factors, postmigration lifestyle changes, or perimenopausal exposures.