Background: The incidence of emergency department (ED) visits by oncology patients has grown in recent years. Some visits are medically unnecessary or could be prevented with improved prophylactic measures. Centers for Medicare & Medicaid Services (CMS) implemented the Hospital Outpatient Quality Reporting (OQR) Program, to which hospitals must report data on quality measures of care furnished in the outpatient setting. One of the proposed measures, OP-35, evaluates admissions and ED visits by patients receiving outpatient chemotherapy, assessing for ten potentially preventable visits. These ten visit reasons included are: anemia, nausea, vomiting, dehydration, neutropenia, diarrhea, pain, pneumonia, fever, and sepsis. This study evaluated the incidence and reasons for ED visits by patients receiving outpatient chemotherapy, as well as outcomes following these visits in order to assess appropriate prophylactic treatment related to outpatient chemotherapy at our institution. Methods: This single-center, retrospective chart review was conducted at The University of Arizona Cancer Center. We collected data using an electronic medical record and included oncology patients with active cancer who had ED visits from January 1, 2018, to December 31, 2018. Data points included baseline demographics; malignancy; ED visit reason; clinic visit history; past ED visits; treatment and supportive care data; and outcome of the ED visit. Results: Study authors included 205 of the 1,659 screened encounters. Approximately 70% of the encounters involved patients with solid tumors and 30% with hematologic malignancies. Nearly 50% of hematologic malignancy patients presented to the ED for potentially preventable reasons, while 32.8% of solid tumor patients had potentially preventable visits. The most common preventable ED visit reasons in both groups were pain, fever, nausea, vomiting, and dehydration. In our analysis, over 90% of patients presenting for neutropenia and fever had received appropriate growth factor prophylaxis when indicated based on their chemotherapy regimen. Of the patients presenting for nausea or vomiting, it was determined 95.3% had received appropriate antiemetic prophylaxis. Conclusions: This study showed that the incidence of potentially preventable visits was numerically higher in the hematologic cancer group compared with the solid tumor group. In addition, this study showed that a number of patients who were admitted to the ED with nausea/vomiting or neutropenia/fever received appropriate guideline-recommended prophylactic or supportive care therapy, regardless of the OP-35 proposed CMS core measure determination that these visit reasons should be considered preventable. These findings highlight the need for CMS to further review OP-35 to include information addressing the use of appropriate supportive care in the future.