Background: Febrile neutropenia (FN) is a serious complication following chemotherapy and is associated with significant mortality and financial expenditure. The aim of this study was to evaluate risk factors for longer length of stay (LOS) and mortality and cost of treatment among hospitalized adults with cancer who developed febrile neutropenia in Thailand. Materials and Methods: Information on illness of inpatients and casualties came from hospitals nationwide and from hospital withdrawals from the 3 health insurance schemes in fiscal 2010. The data covered 96% of the population and were analyzed by age groups, hospital level, and insurance year schemes in patients with febrile neutropenia. Results: A total of 5,809 patients were identified in the study. The mortality rate was 14%. The median LOS was 8.67 days and 69% of patients stayed for longer than 5 days. On bivariate analysis, age, cancer type, and infectious complications (bacteremia/sepsis, hypotension, fungal infections, and pneumonia) were significantly associated with longer LOS and death. On multivariate analysis, acute leukemia and infectious complications were linked with longer LOS and death significantly. The median cost of hospitalized FN was THB 33,686 (USD 1,122) with the highest cost observed in acute leukemia patients. Conclusions: FN in adult patients results in significant mortality in hospitalized Thai patients. Factors associated with increased mortality include older age (>70), acute leukemia, comorbidity, and infectious complications.