In recent years, breast cancer incidence rates have fluctuated over relatively short time spans; examination of these patterns can provide etiologic clues and direction for prevention programs. Asian-American women are generally considered to be at lower risk of breast cancer than other ethnic groups. However, their rates are typically based on an aggregation of ethnic Asian populations, which may obscure important ethnic differences in risk. Detailed analyses of the trends in ethnic-specific incidence rates will provide more information than when ethnicities are combined. Los Angeles County, California, the most populous and probably the most ethnically diverse county in the United States, has a large multi-ethnic Asian-American population. Trends in invasive female breast cancer incidence were examined using data from the Los Angeles Cancer Surveillance Program, the population-based cancer registry covering the County. Athough overall breast cancer incidence rates remained stable in the late 1980s and early 1990s, data for the most recent 5-year period suggest that incidence may again be increasing for Asian-American and non-Hispanic white women over age 50 (estimated annual percent change = 6.3%, p < 0.05 and 1.5%, p < 0.05, respectively), although little change has occurred among black and Hispanic women. Invasive breast cancer incidence rates for Asian-American ethnic groups are heterogeneous and, for most, are increasing. In Los Angeles County, rates for Japanese-American women have increased rapidly since 1988 and are now approaching rates for non-Hispanic white women. Rates among Filipinas, who have historically had higher rates than their other Asian-American counterparts, are not increasing as rapidly as rates for Japanese women, but remain relatively high. Breast cancer risk among women of Japanese and Filipino ancestry is twice that of Chinese and Korean women. Asian women, who commonly have low breast cancer rates in their native countries, typically experience increasing breast cancer incidence after immigrating to the United States. Ethnic-specific incidence rates show that Japanese-Americans, the first Asian population to immigrate to Los Angeles County in large numbers and the most acculturated, have experienced a rapid increase in breast cancer incidence. Japanese-American rates in Los Angeles County may have already surpassed those of non-Hispanic whites if recent trends have continued unabated. © 2002 Wiley-Liss, Inc.