Family caregivers are increasingly recognized as a vital part of the comprehensive treatment of cancer. Many caregivers, especially those caring for patients with primary malignant brain tumor (PMBT), report feeling overwhelmed by providing care. Social support can be protective for caregiving, but there is little research on the composition of social networks of caregivers. The research describes these social networks. Caregivers were recruited from a neuro-oncology clinic at an NCI-designated comprehensive cancer center in the United States from May to August 2017.