As part of a case-control study of the efficacy of screening mammography, the authors validated the mammography histories of 2,495 women aged 40-64 years with incident breast cancer diagnosed in 1994-1998 and a 25% random sample of 615 controls never diagnosed with breast cancer, all reporting a mammogram in the past 5 years. Subjects from five metropolitan areas of the United States were cross-classified by facility records ("gold standard") and self-report according to history of a recent screening mammogram (within 1 year or within 2 years). Sensitivity and specificity of self-reported screening at 1 year were 0.93 and 0.82, respectively, for cases and 0.92 and 0.80 for controls. At 2 years, sensitivity and specificity were 0.97 and 0.78 for both cases and controls. Confidence intervals for the differences in sensitivity and specificity were narrow and included zero. Scant evidence was found of telescoping (recollection of events as more recent than actual). Findings suggest that, in an interview-based case-control study of the efficacy of screening mammography, 1) estimated true prevalences of recent screening mammography adjusted for sensitivity and specificity will be slightly lower than self-reported prevalences, and 2) differential misclassification of exposure status is slight. Therefore, odds ratios will likely be biased toward the null, underestimating screening efficacy.