OBJECTIVE To examine data from advanced cancer patients and their oncologists regarding patient age-related differences in patient and oncologist perspectives on involvement of primary care physicians (PCPs) in aspects of cancer management. METHODS Randomized controlled trial of a support intervention for patients with late-stage cancer treated in two teaching hospital-based cancer clinics caring for underserved populations. METHODS Three hundred fifty-seven patients who had an oncologist and PCP enrolled 2 to 3 months after an advanced cancer diagnosis. METHODS Sociodemographic data and structured interviews were used to assess patients' perceptions of PCP involvement in care discussions and decision-making and satisfaction with that involvement, oncologists' beliefs about how involved PCPs should be in these discussions and decisions, and their shared care practices. RESULTS Older patients (>or=65) were more likely to have a PCP (P=.02). Patients reported a broad range of perceived PCP involvement and satisfaction with that involvement. Greater involvement was associated with greater satisfaction (P<.001). Half of oncologists reported themselves as PCP for more than 25% of their patients. Approximately half of oncologists reported that more older than younger patients had PCPs, yet only 20% reported differences in PCP involvement or in their communication with PCPs for older late-stage patients. CONCLUSIONS Results support involvement of PCPs in advanced cancer care and demonstrate variable perspectives on PCP involvement. Matching patient preferences and practices may improve satisfaction. Clarification of elements in the partnership between patients, PCPs, and oncologists will inform efforts to optimally care for older patients with advanced cancer.