Women who undergo genetic counseling concerning their increased risk of developing breast cancer confront large quantities of complex information in a short period of time. Clinical reports have suggested that many women may not retain what they learned during counseling. A validated questionnaire to measure their knowledge, however, is lacking. In this study, we describe the development and validation of a questionnaire to assess knowledge of information typically included in genetic counseling for breast cancer. Items were empirically derived from detailed content analyses of actual genetic counseling sessions. The instrument's content validity was high, as evidenced by high levels of independent interrater agreement (0.93) on items. Subsequent data reduction and confirmatory factor analytic techniques yielded a highly reliable (alpha = 0.92) 27-item Breast Cancer Genetic Counseling Knowledge Questionnaire (BGKQ). Direct comparison of this questionnaire to a scale previously developed in the literature (BCHK; [Breast Cancer Res. Treat. 53 (1999) 69]) supported the utility of the new questionnaire for evaluation of knowledge after counseling. Compared to non-counseled groups (n = 45), women who had undergone genetic counseling (n = 28) scored significantly higher (P < 0.0001) on the BGKQ, but not on the other questionnaire, establishing the BGKQ's criterion validity. The BGKQ may, thus, provide a useful clinical and research tool for assessing knowledge of information provided during genetic counseling and exploring the potential impact of distress on knowledge, as well as the impact of knowledge on screening behaviors.