Background: Identification of socially vulnerable cancer patients in the health care system is difficult. Only little is known concerning changes in the patients’ social circumstances during the trajectory. Such knowledge is valuable regarding the identification of socially vulnerable patients in the health care system. The objective of this study was to use administrative data to identify population-based characteristics of socially vulnerable cancer patients and investigate how social vulnerability changed during the cancer trajectory. Methodology: A registry-based social vulnerability index (rSVI) was applied to each cancer patient prior to their diagnosis, and used to assess changes in social vulnerability after the diagnosis. Results: A total of 32,497 cancer patients were included. Short-term survivors (n = 13,994) died from cancer from one to three years after the diagnosis, and long-term survivors (n = 18,555) survived at least three years after the diagnosis. 2452 (18 %) short-term survivors and 2563 (14 %) long-term survivors were categorized as socially vulnerable at diagnosis, of these 22 % and 33 % changed category to not socially vulnerable during the first two years after the diagnosis, respectively. For patients changing status of social vulnerability, several social and health-related indicators changed, which is in line with the complexity of the multifactorial social vulnerability. Less than 6 % of the patients categorized as not vulnerable at diagnosis, changed to become vulnerable during the following two years. Conclusion: During the cancer trajectory, social vulnerability may change in both directions. Surprisingly, more patients, who were categorized as socially vulnerable when their cancer was diagnosed, changed status to not socially vulnerable during follow-up. Future research should attempt to increase knowledge on identifying cancer patients, who experience deterioration after the diagnosis.