We sought to assess the reliability of information regarding the maternal history of cancer by comparing the medical records of 214 women with breast cancer, ages 26-59 years and diagnosed in 1974-1995, and of their controls with the medical records of their mothers. Subjects were members of Kaiser Permanente, Northern California, selected for a study of early-life predictors of breast cancer. For any type of cancer identified in the mother’s medical record, the proportion noted in the daughter’s medical record at least 6 months before the daughter’s diagnosis or reference date was 56% among cases and 32% among controls. The odds ratio for the association of maternal cancer history with breast cancer risk was 2.1 using the maternal record and 3.5 using the subject’s record. For a maternal history of breast cancer, the proportion noted in the subject’s record was 79% among cases and 57% among controls, and the odds ratios were 4.0 and 6.5, respectively. We believe that the case-control difference in missing information was attributable to higher utilization of breast cancer screening among cases. This study illustrates the need to assess the impact of screening differences on the ascertainment of information from the medical records.