OBJECTIVES: To examine the effect of informal cancer caregiving and repetitive negative thinking (RNT) on depressive symptoms and salivary cortisol levels. SAMPLE & SETTING: The sample was recruited from a hospital bone marrow unit and caregiver support organizations. It included 60 informal cancer caregivers (52% partners) of individuals with cancer who provided care for a median of 27.5 hours per week for 12 months, and 46 noncaregiver participants. METHODS & VARIABLES: In this cross-sectional study, participants completed questionnaires assessing RNT and depressive symptoms and provided saliva samples to measure cortisol levels. RESULTS: Cancer caregiving and RNT, but not the interaction, were associated with more depressive symptoms. RNT, but not cancer caregiving, was associated with salivary cortisol. A disordinal interaction effect suggests that cancer caregiving was associated with lower cortisol levels, and RNT in noncaregivers was associated with higher cortisol levels. IMPLICATIONS FOR NURSING: Given that RNT is related to depressive symptoms and cortisol, connecting cancer caregivers who experience RNT to resources and the development and evaluation of brief nurse-led interventions to reduce RNT in informal cancer caregivers seems warranted.