BACKGROUND: Shared decision-making is an important component of patient-centered care and is associated with improved outcomes. To the authors' knowledge, little is known concerning the extent and predictors of the involvement of a patient's family in decisions regarding cancer treatments. METHODS: The Cancer Care Outcomes Research and Surveillance (CanCORS) Consortium is a large, multiregional, prospective cohort study of the cancer care and outcomes of patients with lung and colorectal cancer. Participants reported the roles of their families in decision-making regarding treatment. Multinomial logistic regression was used to assess patient factors associated with family roles in decisions. RESULTS: Among 5284 patients, 80 (1.5%) reported family-controlled decisions, with the highest adjusted rates (12.8%) noted among non-English-speaking Asians. Among the 5204 remaining patients, 49.4% reported equally sharing decisions with family, 22.1% reported some family input, and 28.5% reported little family input. In adjusted analyses, patients who were married, female, older, and insured more often reported equally shared decisions with family (all P <.001). Adjusted family involvement varied by race/ethnicity and language, with Chinese-speaking Asian (59.8%) and Spanish-speaking Hispanic (54.8%) patients equally sharing decisions with family more often than white individuals (47.6%). Veterans Affairs patients were least likely to report sharing decisions with family, even after adjustment for marital status and social support (P <.001). CONCLUSIONS: The majority of patients with newly diagnosed lung or colorectal cancer involve family members in treatment decisions. Non-English-speaking Asians and Hispanics rely significantly on family. Further studies are needed to determine the impact of family involvement in treatment decisions on outcomes; until then, physicians should consider eliciting patients' preferences for family involvement.