Aim: To prospectively explore the quality of the relationship between significant others and patients during lung cancer. based on the perceptions of the significant others. Method: In a sample of 91 significant others, longitudinal data were collected during the first year after diagnosis, and explored on group level and as individual patterns over time. Results: Relational quality was skewed towards high quality, although 30% of the significant others reported low levels close to diagnosis. Forty-eight percent reported stability in the quality of their relationship during the disease trajectory. Within this group, 36% reported low levels of relational quality. Fifty-two percent reported change in quality of relationship and four typical patterns of change were identified. Two showed approximate linear changes in either a positive direction (15%) or a negative direction (49%), and two showed non-linear changes with a temporary ascending curve (11%) or a descending curve (26%). This implies that a change towards low levels of relational quality was most common. Conclusion: The present results show that illness may be a trigger for change in relational quality, which may have implications for future family-centred practice and research, since previously high relational quality has been linked to improved emotional well-being.