Background: Trimethoprim-sulfamethoxazole (TMP-SMX) is preferred for prevention of Pneumocystis pneumonia (PCP), toxoplasmosis and other opportunistic infections after allogeneic hematopoietic cell transplantation (HCT), but it is often withheld due to potential adverse events (AE). From July 2016-June 2017 we observed that 31% of allogeneic HCT recipients at our hospital were prescribed TMP-SMX throughout the high risk period (day +30-180). Therefore, we developed an intervention to explore reasons for such a low prescription rate and to optimize TMP-SMX use. Methods: Our institution’s HCT practice guidelines were revised to include indications for TMP-SMX and alternative agents for PCP and Toxoplasma prophylaxis and specific clinical and laboratory criteria for withholding TMP-SMX. Education on the guidelines was provided to transplant clinicians. Outpatient encounters were prospectively audited from January to April 2018. Providers received real-time feedback with recommendations in accordance with our guidelines prior to patients’ visits. Results: We reviewed 321 outpatient encounters among 112 patients. Seventy-six (68%) patients were on TMP-SMX prophylaxis at the first reviewed encounter. Cytopenias were the most common reason for withholding TMP-SMX, occurring in 50 (57%) encounters, followed by elevated liver function tests (LFTs) (13.6%), hyperkalemia (3.4%), elevated creatinine (3.4%), rash or hypersensitivity (3.4%), and unknown/other (22%). In 73 encounters in which patients received alternative or no PCP prophylaxis, we recommended changing to TMP-SMX. This intervention was accepted in 15 patients, 11(73%) of whom tolerated TMP-SMX well. In the remaining four patients, TMP-SMX was subsequently discontinued due to unrelated AE (N = 3) and possible TMP-SMX-associated elevated LFTs (N = 1). Conclusions: Provider education and real-time feedback led to increased utilization of TMP-SMX for prevention of PCP and other opportunistic infections in HCT recipients without an increase in TMP-SMX-related adverse effects. Further study is needed to understand the low clinician acceptance rate of the intervention.