IMPORTANCE: Patients with advanced cancer often report expectations for survival that differ from their oncologists' expectations. Whether patients know that their survival expectations differ from those of their oncologists remains unknown. This distinction is important because knowingly expressing differences of opinion is important for shared decision making, whereas patients not knowing that their understanding differs from that of their treating physician is a potential marker of inadequate communication. OBJECTIVE: To describe the prevalence, distribution, and proportion of prognostic discordance that is due to patients' knowingly vs unknowingly expressing an opinion that differs from that of their oncologist. DESIGN, SETTING, AND PARTICIPANTS: Cross-sectional study conducted at academic and community oncology practices in Rochester, New York, and Sacramento, California. The sample comprises 236 patients with advanced cancer and their 38 oncologists who participated in a randomized trial of an intervention to improve clinical communication. Participants were enrolled from August 2012 to June 2014 and followed up until October 2015. MAIN OUTCOMES AND MEASURES: We ascertained discordance by comparing patient and oncologist ratings of 2-year survival probability. For discordant pairs, we determined whether patients knew that their opinions differed from those of their oncologists by asking the patients to report how they believed their oncologists rated their 2-year survival. RESULTS: Among the 236 patients (mean [SD] age, 64.5 [11.4] years; 54% female), 161 patient-oncologist survival prognosis ratings (68%; 95% CI, 62%-75%) were discordant. Discordance was substantially more common among nonwhite patients compared with white patients (95% [95% CI, 86%-100%] vs 65% [95% CI, 58%-73%], respectively; P = .03). Among 161 discordant patients, 144 (89%) did not know that their opinions differed from that of their oncologists and nearly all of them (155 of 161 [96%]) were more optimistic than their oncologists. CONCLUSIONS AND RELEVANCE: In this study, patient-oncologist discordance about survival prognosis was common and patients rarely knew that their opinions differed from those of their oncologists.