Abstract We sought to explore the intrafamilial communication and cascade genetic testing (CGT) experiences of patients with hereditary cancer from diverse, medically underserved populations and their relatives. Participants included patients receiving oncology care at an urban, safety net hospital in Texas or comprehensive cancer center in Alabama and their first-degree relatives. In-depth semi-structured qualitative interviews were completed wherein patients shared their experiences with genetic counseling (GC), genetic testing (GT), and communicating their results to relatives. Relatives shared their experiences receiving information from the patient and considering CGT. Interviews were transcribed, coded, and themes were identified. Of 25 participating patients, most recalled key aspects of GC and their GT results. Most (80%) patients shared their results with relatives, but only some relatives underwent CGT; patients reported low perceived susceptibility to hereditary cancer as a common barrier to CGT for their relatives. Of 16 participating relatives, most reported feeling distress upon learning the patient's GT results. Relatives were fearful of learning their own CGT results but identified prevention and early detection as CGT benefits. Interviews identified opportunities during family communication to improve relatives’ perceived susceptibility to hereditary cancer. Tailored resources may support patients and relatives experiencing distress and fear during GT. Prevention Relevance: This study of intrafamilial communication and cascade genetic testing experiences of patients with hereditary cancer and their relatives from diverse, medically underserved populations identified relatives’ perceived susceptibility to hereditary cancer risks, distress, and fear as frequent reactions and barriers to testing. These results may inform future hereditary cancer prevention efforts.