Purpose: The present study examined the degree to which loneliness mediated the influence of negative (social constraints) and positive (emotional support) relationship qualities on the global mental health of advanced gastrointestinal (GI) cancer patients and their family caregivers. Methods: Fifty patient-caregiver dyads completed measures assessing social constraints (e.g., avoidance, criticism) from the other dyad members, emotional support from others, loneliness, and global mental health.