BACKGROUND AND OBJECTIVES Failure or delay to diagnose brain death leads to needless occupation of a hospital bed, emotional and financial losses, and unavailability of organs for transplants. The intensive care physician plays an essential role in this diagnosis. This study intended to evaluate intensivists' knowledge concerning brain death. METHODS Cross-sectional study in 15 intensive care units (ICU) in eight hospitals in the city of Porto Alegre, Brazil. RESULTS Two hundred forty-six intensivists were interviewed in a consecutive sample between April and December 2005. The prevalence of lack of knowledge regarding the concept was of 17%. Twenty per cent of the interviewees ignored the legal need for complementary confirmatory tests for their diagnosis. Forty-seven per cent considered themselves as having the highest level of assurance to explain the concept to a patient's family members. Twenty-nine per cent erroneously determined the legal time of death for brain dead patients. Pediatric intensivists had less knowledge about the concept, when compared to intensivists for adults (p < 0.001). CONCLUSIONS Current knowledge of brain death is insufficient in Brazil, among the health care professionals who most often find patients in this situation. Education on the subject is needed to avoid unnecessary expenses, reduce family suffering and increase the offer of organs for transplant.