Objective: Neutropenia is a major adverse event often associated with chemotherapy administration. The purpose of this study was to evaluate costs for chemotherapy-related neutropenia (N) and febrile neutropenia (FN) in an elderly population with advanced NSCLC. Methods: Study patients included those aged 65 years and older with a diagnosis of Stage IIIB/IV NSCLC in the SEER cancer registry from 1998 through 2002. Neutropenia patients were followed in the SEER-Medicare database to evaluate the costs associated with N and FN. Neutropenia was identified by the presence of a primary or secondary ICD-9-CM 288.x diagnostic code during a period of chemotherapy treatment. FN was defined by an inpatient hospitalization for neutropenia or antibiotic administration following the initial neutropenia diagnosis. All Medicare payments were summed for two main types of cost measures: neutropenia-related costs and costs unrelated to neutropenia. Costs were classified using ICD-9-CM diagnosis and procedure codes appearing on the claims, with confidence intervals for all cost measures estimated by using non-parametric bootstrapping techniques. Results: Among elderly patients treated first-line for advanced NSCLC, 5138 were identified who met inclusion criteria. Nearly one-quarter (N=1228) developed N or FN while on first-line chemotherapy. Mean first-line neutropenia-related costs for FN were $11,661 (95% CI: 10,493-13,098) compared to $2,817 (95% CI: 2,522-3,124) for N, with length of follow-up of 134 and 165 days respectively. Costs related to neutropenia for FN patients accounted for 31% of total first-line costs compared to only 8% for N. Neutropenia-related costs during first-line were $2000 higher per month for FN when compared to N. Costs unrelated to neutropenia were comparable between N and FN when adjusted for variable follow-up. Conclusion: Neutropenia, and in particular FN, add additional costs to first-line chemotherapy treatment in elderly advanced NSCLC patients.