BACKGROUND: Vietnamese-American women underutilize breast cancer screening. DESIGN: An RCT was conducted comparing the effect of lay health workers (LHWs) and media education (ME) to ME alone on breast cancer screening among these women. SETTING/PARTICIPANTS: Conducted in California from 2004 to 2007, the study included 1100 Vietnamese-American women aged > or = 40 years who were recruited through LHW social networks. Data were analyzed from 2007 to 2009. INTERVENTION: Both groups received targeted ME. The intervention group received two LHW educational sessions and two telephone calls. MAIN OUTCOME MEASURES: Change in self-reported receipt of mammography ever, mammography within 2 years, clinical breast examination (CBE) ever, or CBE within 2 years. RESULTS: The LHW+ME group increased receipt of mammography ever and mammography in the past 2 years (84.1% to 91.6% and 64.7% to 82.1%, p<0.001) while the ME group did not. Both ME (73.1% to 79.0%, p<0.001) and LHW+ME (68.1% to 85.5%, p<0.001) groups increased receipt of CBE ever, but the LHW+ME group had a significantly greater increase. The results were similar for CBE within 2 years. In multivariate analyses, LHW+ME was significantly more effective than ME for all four outcomes, with ORs of 3.62 (95% CI=1.35, 9.76) for mammography ever; 3.14 (95% CI=1.98, 5.01) for mammography within 2 years; 2.94 (95% CI=1.63, 5.30) for CBE ever; and 3.04 (95% CI=2.11, 4.37) for CBE within 2 years. CONCLUSIONS: Increased breast cancer screening by LHWs among Vietnamese-American women. Future research should focus on how LHWs work and whether LHW outreach can be disseminated to other ethnic groups.