OBJECTIVES: Examine the effectiveness of using prophylactic G-CSF among NHL patients treated with anthracycline-based chemotherapy in practice.  METHODS: Using the national Surveillance, Epidemiology and End Results (SEER)-Medicare linked database, we studied patients 66 years or older diagnosed as NHL and on anthracycline in one of the 13 SEER registry areas from 1994-2002. Prophylactic G-CSF use was designated if a patient had a G-CSF claim within the first 5 days of the first chemotherapy cycle. Neutropenia hospitalization (NH) was identified within 6 months of diagnosis. Multiple regression estimates were used to examine whether treated patients actually benefited. Instrumental variable estimates using local area prophylactic G-CSF treatment rates as instruments were used to estimate whether increases in the G-CSF utilization rate could lead to further reductions in the rate of neutropenic hospitalization. RESULTS: Only 9.83% of study patients had early G-CSF. After adjustment for patient demographic and clinical risk factors, multiple regressions indicated prophylactic G-CSF significantly reduced NH events for the patients who received G-CSF (OR=0.54, 95% CI=0.34-0.87).  Chow F-statistics showed our instrumental variable described a statistically significant portion in the variation in G-CSF use (F=14.65, P=0.0001). Our instrumental variable estimates were not statistically significant from zero.  CONCLUSIONS: Among elderly NHL patients on anthracycline-based chemotherapy, our multiple regression estimates suggest that prophylactic G-CSF treated patients reduced their neutropenia risk. However, our instrumental variable results suggest that expanding prophylactic G-CSF use rates will not significantly reduce NH rates. These results suggest that providers optimally sorted the use of G-CSF patients to those patients that are most apt to gain.