OBJECTIVES: Recent studies have established aging as a risk factor of chemotherapy-induced neutropenia. Thus, the 2006 ASCO guideline added prophylactic use of CSF in older cancer patients to its recommendations. Our study examined current patterns of CSF use in outpatient settings in this population. METHODS: We selected cancer patients with chemotherapy from the 2001-2004 Medicare MarketScan data, a proprietary dataset collecting Medicare and commercial claims for a group of retirees with Medicare supplemental insurance. We defined the index date as the first date of chemotherapy, followed patients for 60 days, and classified high-risk patients as those whose chemotherapy regimens had a greater than 40% risk of neutropenia. We identified outpatient CSF use using HCPCS codes and conducted logistic regression to examine factors associated with CSF use; factors included age, gender, cancer types, risk class, geographic regions, and the index year. A subgroup analysis of the high-risk patients was also performed. RESULTS: We found CSF use in 8.71% (5,455 in 62,647) of the patients and 19.8% (754 in 3,812) among high-risk patients. Compared with patients in the age group 65-69, those in 75-79 and 80+ groups were significantly less likely to use CSF (OR=0.83 CI: 0.76-0.90; OR=0.73, CI: 0.66-0.79, respectively). High-risk patients were more likely to use CSF (OR=1.12, CI: 1.02-1.24), so were those treated in more recent years. Compared with lung cancer patients, breast cancer and lymphoma patients were 1.23 (CI: 1.15-1.41) and 1.81 (CI: 1.64-2.00) times more likely to use CSF. Subgroup analysis showed that patients 75 and over were significantly less likely (OR = 0.83, CI: 0.70-0.99) to use CSF than those under 75. CONCLUSION: The observed lower rate in CSF use in the older age groups points to an unmet supportive care need in a subgroup of patients who are more susceptible to neutropenia due to aging.