Background: Avoiding acute care services can improve cancer care and reduce cost. The US Centers for Medicare and Medicaid Services’ (CMS) new oncology outcome measure (OP-35) defines 30-day post-chemotherapy inpatient (IP) and/or emergency department (ED) events (IP/ED) as “potentially avoidable” if involving any of 10 toxicities, including nausea or vomiting (NV). Evidence demonstrates meaningful gaps in oncologists’ adherence to antiemetic prophylaxis guidelines for highly emetogenic chemotherapy (HEC), and that NV-related IP use costs >$10,000; yet the incidence of avoidable acute care events involving NV is not well studied. Methods: We assessed chemotherapy courses using IBM Explorys electronic health records (4Q 2012–1Q 2018). We identified rates of IP/ED ≤30 days post-chemotherapy, and OP-35 toxicities (NV, anemia, dehydration, diarrhea, fever, neutropenia, pain, pneumonia, or sepsis) by ICD-9, ICD-10, procedure codes, and CMS criteria. We evaluated cisplatin, anthracycline + cyclophosphamide (AC), carboplatin (>14 days apart, as a proxy for AUC ≥4), oxaliplatin (OX), and other non-HEC chemotherapy. We assessed guideline adherence, defined as triple prophylaxis (NK1 RA + 5HT3 RA +dexamethasone) rates at HEC initiation. Results: In 17,609 HEC and 56,624 non-HEC courses, we observed 30-day IP/ED utilization in 29% and 19% of courses, respectively (Table 1). For HEC, 76% of IP/ED use involved ≥1 of the 10 CMS toxicities, most often anemia (42%), pain (41%), dehydration (24%), and NV (24%). Rates of all-cause IP/ED, IP/ED with OP-35 toxicity, and NV-related IP/ED were consistent for HEC and OX. Gaps in triple prophylaxis were common in HEC. Conclusion: Roughly one-third of patients receiving HEC or OX experienced IP/ED events ≤30 days after chemotherapy. Three-quarters of IP/ED events involved ≥1 of 10 OP-35 toxicities linked by CMS to potentially avoidable acute care; of these, one-third involved NV. NV-associated acute care use is considerable, costly, and potentially avoidable with better adherence to antiemesis guidelines.