Background Despite the important role that family caregivers play in providing emotional and practical support to cancer patients, relatively little is known about the family caregiver's role in treatment decision‐making (TDM). We sought to investigate patients' and family caregivers' preferences for and experiences of family involvement in TDM and factors associated with preference concordance. Method A national survey was performed with 990 patient–caregiver dyads (participation rate:76.2%). Questions examining preferences for and experiences of family involvement in TDM were administered independently to patients and family caregivers. Concordance was calculated with weighted kappa. Logistic regression analyses were used to identify predictors of patients' and caregivers' preferences for family involvement in TDM and concordance between them. Results Few patients or family caregivers expressed a preference for unilateral decision‐making; however, there was considerable variation and poor agreement within dyads in regard to whether the patient or family caregivers should take the lead in decision‐making with input from the other (weighted kappa between respondents for TDM preferences and experiences = 0.10 and κ = 0.18, respectively). Greater TDM preference concordance was associated with higher patient education, whereas lower levels of concordance were evident for younger patients, less educated caregivers, adult child patient dyads (as opposed to a spouse–patient dyads) and problematic family communication about cancer. Conclusions Most patients and family caregivers valued and expected family involvement in TDM. However, there is little explicit agreement in regard to which party in the dyad should take decisional leadership and who should play a supporting role. Copyright © 2013 John Wiley & Sons, Ltd.