Hemophagocytic lymphohistiocytosis (HLH) is a rare clinical syndrome characterized by the activation of the mononuclear phagocytic system. The diagnosis of HLH in adults is challenging not only because the majority of the reported data are from pediatric patients, but also because HLH occurs in many disease entities. This study reports the clinical and laboratory findings and prognostic factors of adult HLH in a large cohort managed at a single medical center from 2003 to 2014. Seventy-three patients met the HLH-2004 diagnostic criteria. The median age was 51 years (range, 18-82 years); 41 (56.2%) were male. Patients manifested fever, cytopenias, and elevated ferritin in >85% of cases. Likely causes of HLH were as follows: 30 (41.1%) infections, 21 (28.8%) malignancies, 5 (6.8%) attributed to autoimmune disorders, 1 (1.4%) primary immunodeficiency, 2 (2.7%) post solid organ transplantation, and 13 (17.8%) idiopathic. The median overall survival was 7.67 months. Patients with malignancy-associated HLH had a markedly worse survival compared with patients with non-malignancy-associated HLH (median overall survival 1.13 vs. 46.53 months, respectively; P<0.0001). In a multivariable analysis, malignancy (hazard ratio=12.22; 95% CI: 2.53-59.02; P=0.002) correlated with poor survival. Ferritin >50,000 μg/L correlated with 30-day mortality. Survival after a diagnosis of HLH is dismal, especially among those with malignancy-associated HLH. The development of a registry for adults with HLH would improve our understanding of this syndrome, validate diagnostic criteria, and help develop effective treatment strategies.