Background: Both advanced cancer patients and their family caregivers experience distress and have a range of concerns after cancer diagnosis. However, longitudinal studies on this topic have been lacking. Aim: To investigate concerns in both patients with advanced lung cancer and their family caregivers longitudinally from diagnosis. Design: A multi-center prospective questionnaire-based study. Setting/participants: We recruited patients with newly diagnosed advanced lung cancer and their family caregivers at 16 hospitals in Japan. We prospectively assessed the prevalence of their concerns using the Concerns Checklist and investigated the associations between their concerns and mental status as well as quality of life until 24 months after diagnosis. Results: A total of 248 patients and their 232 family caregivers were enrolled. The prevalence of serious concerns was highest at diagnosis (patients: 68.3%, family caregivers: 65.3%). The most common serious concern was concern about the future in both groups at diagnosis (38.2% and 40.5%, respectively) and this remained high in prevalence over time, while the high prevalence of concern about lack of information improved 3 months after diagnosis in both groups. Approximately one-third of patient-family caregiver dyads had discrepant reports of serious concerns. The presence of serious concerns was significantly associated with anxiety and depression continuously in both groups. Conclusions: The majority of advanced lung cancer patients and their family caregivers have serious concerns from diagnosis, which is associated with their psychological distress. The spectrum of concerns alters over the disease trajectory, warranting efficient tailored care and support for both groups immediately after diagnosis.