Comparisons of symptom ratings and health-related quality of life between significant others and patients have been the focus of numerous studies during the past decades. Additional studies are needed to assess the discrepancies identified in this work. In the present cross-sectional exploratory study, focus has been on evaluating the accuracy of significant other proxy ratings and on investigating factors that influence agreement between lung cancer patients and significant others based on dyadic assessments from 52 patients and 54 significant others. Results indicate that the levels of agreement are fair to good, but that significant others consistently rate the patients' symptoms higher and functioning lower than the patients do themselves. Factors found to influence agreement in various dimensions of symptoms and functioning were gender, patient age, and significant others' self-reported lack of family support, health problems, and caregiver esteem.