BACKGROUND: Screening mammogram and Pap smear rates are lower for women in underserved racial and ethnic groups, yet may be overestimated due to reliance on patients’ self-reports. The purpose of this study was to determine accuracy of self-reports of mammograms and Pap smears in a multiethnic, multilingual population of African American, Latina, Chinese, Filipina, and White women residing in low-income census tracts of Alameda County, California. METHODS: Following a baseline telephone survey of 1,464 women regarding receipt of mammograms and Pap smears, we examined computerized and written medical records to validate the dates and locations of tests reported by women. RESULTS: Of 1,464 subjects, 94.9% reported having had a Pap smear, and 87% reported having had a mammogram. For Pap smears, in a subsample of 448 cases, we validated only 69.4% of the women’s self-reports, and for mammography, in a subsample of 846 women, we validated only 75.4% of the self-reports. Validation rates differed significantly by ethnicity and site of care for both Pap smears and mammograms. CONCLUSIONS: Population estimates of breast and cervical cancer screening rates based upon patient self-reports need to be adjusted downward, by as much as one-quarter to one-third, for low-income, ethnic women.