Background: Shared decision making is promoted to improve cancer care quality. Patients and clinicians may have different ideas about what constitutes a cancer care decision, which may limit the validity of self-reported measures of shared decision making. We sought to estimate the extent to which patients and clinicians agree on whether a cancer care decision was made during an outpatient encounter. Methods: We surveyed patients and clinicians immediately after an oncology encounter at a large, tertiary medical centre and calculated agreement in response to the single-item question, 'Was a specific decision about cancer care made during the appointment today?' Answer options were 'yes' and 'no'. Participants were 315 oncology patients, with any solid tumour malignancy and at any stage of management, and their clinicians (22 staff oncologists, nine senior fellows and five nurse practitioners). Results: Patients and clinicians reported having made a cancer care decision in 184 (58%) and 174 (55%) of encounters, respectively. They agreed on whether a cancer care decision was made in 213 (68%) of encounters (chance-adjusted agreement was 0.34); in 56 of the 102 discordant encounters, the patient reported making a decision while the clinician did not. We found no significant correlates with discordance. Conclusions: Patients and clinicians do not always agree on whether a cancer care decision was made. As such, measures that ask patients and/or clinicians to evaluate a decision-making process or outcome may be methodologically insufficient when they do not explicitly orient respondents towards the thing they are being asked to assess.