Insecure adult attachment patterns have recently been associated with an increased risk of developing persistent post-concussion symptoms (PCS) and poorer treatment outcomes, highlighting the potential of attachment-informed treatment protocols for persistent PCS. A key requirement for such approaches is that attachment patterns remain relatively stable during the post-injury illness course, allowing them to serve as reliable indicators for guiding initial care and treatment planning. This study aimed to assess the stability of self-reported attachment anxiety and attachment avoidance in individuals aged 15-30 years, from three (T1) to nine (T2) months after a concussion, corresponding to the period when treatment for PCS typically becomes relevant. This study is a methodological follow-up of a larger clinical cohort study comprising 3080 individuals aged 15-30 years who were diagnosed with a concussion. Attachment dimensions were measured using the Experiences in Close Relationships-Relationship Structures Questionnaire at three (T1) and nine (T2) months post-injury. Both mean-level and individual changes were examined, taking relevant covariates such as sex, age, level of PCS, post-traumatic stress disorder symptoms, and social support into account. At T1, 958 respondents were included (39%), with 416 also responding at T2 (43%). Mean-level changes were minimal (attachment anxiety: β = -0.07 (95% confidence interval [CI]: -0.22
0.08)), attachment avoidance: β = 0.00 (95% CI: -0.11
0.11). Larger individual changes that could not be explained by measurement error were observed in 12% of participants. No association was found with any of the covariates. Self-reported attachment patterns appear to remain stable in most young people with a concussion from 3 to 9 months post-injury. The findings strengthen confidence in conducting research to investigate whether attachment-informed treatment approaches can improve healthcare for these patients.