Background: Patient values (who/what matters most) are integral to cancer treatment decision-making, yet which factors matter most to family caregivers (FCG) remains understudied. To address this gap, we examined differences in factors considered very important by FCGs during cancer treatment decision-making by cancer stage and by FCG sociodemographics. Methods: Analysis of an internet-based 2021 U.S. national survey data of cancer FCGs (N=1,661) conducted by CancerCare. Factors were assessed using the following item: "When you were participating in making this [treatment] decision, how important to you were these factors regarding the person with cancer?" Fifteen factors were rated on a 4-point Likert scale from ‘very unimportant’ to ‘very important’, including patient-focused factors (e.g., quality of life, length of life, functional independence), caregiver-focused factors (e.g., impact on own daily life, own religious/spiritual beliefs), and other-focused factors (e.g., opinions/feelings of other family members/friends). Descriptive statistics and proportions were tabulated for each factor. Generalized linear mixed effect models tested factor importance endorsement by cancer stage (in remission vs. stages I/II vs. stages III/IV) and chi-square analyses determined associations between FCG sociodemographics and each of the most frequently endorsed factors, with adjustment for multiple inferences. Results: The five most important factors endorsed by FCGs were the patient’s quality of life (69%), physical wellbeing (68%), length of life (66%), emotional wellbeing (63%), and opinions/feelings of oncology team (59%). In factor endorsement comparisons by cancer stage, patient’s physical well-being, length of life, ability to continue working, ability to care for others, and the opinions/feelings of the oncology team were more often endorsed as very important by FCGs in the context of early-stage cancers (all p’s <0.05). FCG age, race, gender, and ethnicity had small, significant associations with endorsement of different factors as ‘very important’, most especially towards “opinions/feelings of the oncology team” (all p’s <0.05). Conclusions: Cancer FCGs identified patient’s quality of life, physical wellbeing, and length of life as the most important factors when partnering with their care recipients in cancer treatment decision-making. Several factors were perceived as less important in the context of later stage cancers, and there were notable differences in factors by FCG sociodemographic characteristics, particularly towards the opinions/feelings of the oncology team. Future decision support work is needed to identify the best timing (i.e., prior to vs. during clinic visits) and method (i.e., alone vs. facilitated) to elicit what matters most to patients and FCGs given existing clinical practice demands.