Little is known about the behavioral determinants that underlie cancer patients' intention to express concerns during a consultation. This information can be relevant to developing effective interventions for cancer patients. In this study, the integrative model of behavioral prediction (IMBP) is used as a framework to unravel the determinants of patients' intention to express concerns. The objectives of this study are to examine which of the IMBP determinants (attitude, perceived social norm, and/or self-efficacy) are significantly related to intention and what content of these determinants can be targeted to effect a change in patients' intention. An online survey based on the IMBP determinants was distributed. A total of 236 cancer patients and cancer survivors participated. The results of the survey showed that patients' attitudes and perceived social norm were the most important determinants of their intention to express concerns. The largest change in patients' intention might be achieved by targeting the affective attitude, referring to the extent to which patients believe that expressing concerns is (un)pleasant, and the social norm, referring to the extent to which patients feel (un)supported by significant others in expressing concerns.