The objective of this study was to validate reports on the bilaterality status of breast cancer in first-degree relatives of women with a strong family history of the disease; i.e., women with 1) two first-degree relatives who have, or have had, breast cancer; 2) one first- and one second-degree affected relative; or 3) one first-degree relative with diagnosis of breast cancer before the age of 50 years and/or bilateral breast cancer. We were able to obtain hospital records for 94 affected relatives of 83 patients who agreed to participate in the study. The accounts of these women were compared to the bilaterality status indicated in the hospital records of the affected relatives. Inconsistencies that might have been attributed to incomplete medical records were resolved through personal interviews with the participants, and when indicated, with other family members or the physician of the affected relative. Overall, 89.4% (84/94) of the reports validated in this manner were correct. Participants who reported unilateral breast cancer in a first-degree relative were correct 94.4% (68/72) of the time. Similarly, 94.0% (47/50) of the accounts concerning affected living relatives were accurate, regardless of whether the participant had indicated unilateral or bilateral disease. However, participants who reported bilateral breast cancer in a deceased relative were accurate only 61.5% (8/13) of the time. Incorrect reports were associated with misunderstanding of medical terminology, especially if the participant was young at the time of the diagnosis of her relative.