Fever persisting despite adequate antimicrobial therapy for endocarditis can be an ominous sign. To evaluate the significance of persistent fever in this situation, we reviewed the records of patients at three hospital affiliates of Albert Einstein College of Medicine. Twenty-six patients with 27 episodes of endocarditis and fever lasting for > or = 2 weeks despite appropriate antimicrobial therapy were identified and compared with a matched cohort of 26 patients with endocarditis but without prolonged fever. The median duration of fever in the former group was 35 days. Cardiac infection caused fever in 13 of these patients, seven of whom had myocardial abscesses. Additional causes of infection included drug treatment, nosocomial transmission of pathogens, and pulmonary emboli. Sixteen patients required cardiac surgery (seven on an emergent basis), whereas only two controls underwent such a procedure (P < .001). Twenty-two patients with persistent fever and five controls developed nosocomial complications (P < .001). Six patients with fever died, five from endocarditis-related complications. Thus persistent fever often indicates complicated endocarditis. We present an approach for the evaluation of the patients affected by this condition.