BACKGROUND: In most emergency situations or severe illness, patients are unable to consent for clinical trial enrollment. In such circumstances, the decision about whether to participate in a scientific study or not is made by a legally designated representative. OBJECTIVE: To address the willingness of patients admitted to the intensive care unit (ICU) to be enrolled in a scientific study as volunteers, and to assess the agreement between patients' and their legal representatives' opinion concerning enrollment in a scientific study. METHODS: This survey was conducted in two hospitals in São Paulo, Brazil. Patients (≥18 years) with preserved cognitive functions accompanied by a surrogate admitted to the ICU were eligible for this study. A survey containing 28 questions for patients and 8 questions for surrogates was applied within the first 48h from ICU admission. The survey for patients comprised three sections: demographic characteristics, opinion about participation in clinical research and knowledge about the importance of research. The survey for legal representatives contained two sections: demographic characteristics and assessment of legal representatives' opinion in authorizing patients to be enrolled in research. RESULTS: Between January 2017 and May 2018, 208 pairs of ICU patients and their respective legal representatives answered the survey. Out of 208 ICU patients answering the survey, 73.6% (153/208) were willing to be enrolled in the study as volunteers. Of those patients, 65.1% (97/149) would continue participating in a research even if their legal representative did not support their enrollment. Agreement between patients' and surrogates' opinion concerning participation was poor [Kappa = 0.11 (IC95% -0.02 to 0.25)]. If a consent for study participation had been obtained, 69.1% (103/149) of patients would continue participating in the study until its conclusion, and 23.5% (35/149) would allow researchers to use data collected to date, but would withdraw from the study on that occasion. CONCLUSION: The majority of patients admitted to the ICU were willing to be enrolled in a scientific study as volunteers, also after a deferred informed consent procedure has been used. Nevertheless, contradictory opinions between patients and their and their legal representatives' concerning enrollment in a scientific study were often observed.