OBJECTIVES: We estimated trends in breast cancer incidence rates for specific Asian populations in California to determine if disparities exist by immigrant status and age. METHODS: To calculate rates by ethnicity and immigrant status, we obtained data for 1998 through 2004 cancer diagnoses from the California Cancer Registry and imputed immigrant status from Social Security Numbers for the 26% of cases with missing birthplace information. Population estimates were obtained from the 1990 and 2000 US Censuses. RESULTS: Breast cancer rates were higher among US- than among foreign-born Chinese (incidence rate ratio [IRR] = 1.84; 95% confidence interval [CI] = 1.72, 1.96) and Filipina women (IRR = 1.32; 95% CI = 1.20, 1.44), but similar between US- and foreign-born Japanese women. US-born Chinese and Filipina women who were younger than 55 years had higher rates than did White women of the same age. Rates increased over time in most groups, as high as 4% per year among foreign-born Korean and US-born Filipina women. From 2000-2004, the rate among US-born Filipina women exceeded that of White women. CONCLUSIONS: These findings challenge the notion that breast cancer rates are uniformly low across Asians and therefore suggest a need for increased awareness, targeted cancer control, and research to better understand underlying factors.