The distribution of breast cancer molecular subtypes has been shown to vary by race/ethnicity, highlighting the importance of host factors in breast tumor biology. We undertook the current analysis to determine population-based distributions of breast cancer subtypes among six ethnic Asian groups in California. We defined immunohistochemical (IHC) surrogates for each breast cancer subtype among Chinese, Japanese, Filipina, Korean, Vietnamese, and South Asian patients diagnosed with incident, primary, invasive breast cancer between 2002 and 2007 in the California Cancer Registry as: hormone receptor-positive (HR+)/HER2- [estrogen receptor-positive (ER+) and/or progesterone receptor-positive (PR+), human epidermal growth factor receptor 2-negative (HER2-)], triple-negative (ER-, PR-, and HER2-), and HER2-positive (ER±, PR±, and HER2+). We calculated frequencies of breast cancer subtypes among Asian ethnic groups and evaluated their associations with clinical and demographic factors. Complete IHC data were available for 8,140 Asian women. Compared to non-Hispanic White women, Korean [odds ratio (OR) = 1.8, 95% confidence interval (CI) = 1.5-2.2], Filipina (OR = 1.3, 95% CI = 1.2-1.5), Vietnamese (OR = 1.3, 95% CI = 1.1-1.6), and Chinese (OR = 1.1, 95% CI = 1.0-1.3) women had a significantly increased risk of being diagnosed with HER2-positive breast cancer subtypes after adjusting for age, stage, grade, socioeconomic status, histology, diagnosis year, nativity, and hospital ownership status. We report a significant ethnic disparity in HER2-positive breast cancer in a large population-based cohort enriched for Asian-Americans. Given the poor prognosis and high treatment costs of HER2-positive breast cancer, our results have implications for healthcare resource utilization, cancer biology, and clinical care.