OBJECTIVE Nonbacterial thrombotic endocarditis (NBTE) is characterized by sterile platelet and fibrin deposition on cardiac valves. In this study, our aim was to investigate the clinicopathologic characteristics of patients diagnosed with NBTE at the 12 de Octubre University Hospital in Madrid, Spain. METHODS Patients were identified by analyzing reports of autopsies carried out at the Anatomical Pathology Department of the University Hospital between 1974 and 1977 and between 1980 and 2004. RESULTS Out of 2041 autopsies, 22 (1.08%) showed evidence of NBTE. The patients' average age was 63.04 years, and neither sex predominated. The underlying disease was an infection in 54.5% of cases, whereas 31.8% had a neoplasm, most frequently a pancreatic adenocarcinoma or renal carcinoma. In 10 cases, there was a previous valvulopathy. The mitral valve was most frequently affected by NBTE (36.5% of cases). In most cases, the cause of death was the underlying disease, though in two patients it was directly related to NBTE (9.1%). In nine cases, an embolism occurred, principally in the spleen, the kidneys or the central nervous system. Before death, there was no suspicion of NBTE in any of these cases. CONCLUSIONS NBTE is an uncommon disease entity that occurs not only in patients with end-stage neoplastic disease, but also in those with potentially treatable diseases. Consequently, we should raise our level of awareness of this condition so that a diagnosis can be made before the patient dies and anticoagulant treatment can be started, thereby helping to improve prognosis.