Objective Despite the significant role played by cancer patients' families in medical decision‐making in Asian countries, inconsistencies have hitherto not been evaluated between patients' and families' preferences and doctors' actual practices with regard to cancer truth telling. Methods For this quantitative comparative study of cancer patients' and families' truth‐telling preferences and their experiences of doctors' practices, 532 patients, 551 family members, and 127 doctors (N = 1 210) were enrolled from five hospitals across Taiwan over 2 years. Truth telling was assessed using the Taiwanese version of a modified Japanese truth‐telling scale. Results Patients' truth‐telling preferences and their experiences of doctors' truth‐telling practices differed significantly in scores on the overall truth‐telling scale and each subscale, including method of disclosure, emotional support, additional information, and setting (P < .001). Similar findings were obtained for families' preferences and doctors' actual practices (P < .001). Patients' and families' truth‐telling preference scores were higher than doctors' actual practice scores. Multiple regression analysis revealed a dose‐dependent effect of doctors' monthly truth‐telling frequency on their truth‐telling preferences, but this effect was only borderline significant (P = .08). This multiple regression model explained 30% of the total variance in doctors' truth‐telling preferences (F = 1.38, P = .22). Conclusions Taiwanese medical educational policies need to be revised to better equip doctors to practice truth telling in accordance with the preferences of cancer patients and families. Communication skills training should be prioritized for doctors who refrain from truth telling in actual practice.