Purpose: National guidelines do not include routine pGCSF as primary prophylaxis (PP) for patients receiving chemotherapy associated with a low risk for febrile neutropenia (FN). Inappropriate pGCSF can increase patient morbidity, financial burden, and overall health care costs. In 2013, an interdisciplinary group at TCI implemented a QI project to reduce inappropriate PP pGCSF in patients with lung cancer; this included prescriber education and modification of chemotherapy orders by risk of FN in the electronic medical record (EMR). Inappropriate pGCSF was reduced from 28% to 4%. In this 5-year follow up study we analyzed pGCSF use in lung cancer patients. Methods: We conducted a review of lung cancer patients who received pGCSF with chemotherapy initiated between January 2016 and August 2018. PP pGCSF use was appropriate if prescribed with chemotherapy regimens with a high risk (>20%) for FN, or intermediate risk (10%–20%) if other accepted FN risk factors were present. PP use with FN low-risk (<10%) chemotherapy was considered inappropriate. Treating physicians were anonymously surveyed about their practices. Results: 294 patients with lung cancer received 1,353 doses of pGCSF during the study period. 58 (20%) were treated at TCI by subspecialty thoracic oncologists and 236 (80%) were treated at regional network sites. 100/294 (34%) patients received low-risk regimens. 62/100 (62%) patients treated with low-risk regimens received 311 doses of PP pGCSF (inappropriate use). 5/62 (8%) of inappropriate use occurred at TCI; 57/62 (92%) at network sites. Of 130 patients who received an intermediate risk regimen, 99 (76%) received PP pGCSF. At least one risk factor for FN was identified in 80/99 (80%) of these patients; age >65 and prior chemotherapy or radiation were the top-cited factors. 33/294 (11%) patients were hospitalized for FN during the study period; 7/100 (7%) received low-risk regimens, 15/130 (11.5%) intermediate-risk, and 11/46 (24%) high-risk regimens. All physicians responding to the survey indicated awareness of guidelines and EMR risk identification. Conclusion: After initial success at our center, we found that guideline-based alignment of pGCSF prescribing in lung cancer patients was not sustained. Despite reported familiarity with guidelines for PP pGCSF use, this analysis suggests an opportunity for re-education and further EMR modification. Based on July 2018 CMS average sales price, reduction in inappropriate use presents a potential cost savings of $1.5 million during the study.