PURPOSE: After ASCO and National Comprehensive Cancer Network guideline recommendations for triple antiemetic prophylaxis for carboplatin area under the curve (AUC) ≥ 4, and the publication of studies documenting avoidable acute care after chemotherapy involving nausea and vomiting (NV) and other toxicities, we studied clinician adherence to the guideline change and assessed avoidable acute-care use. METHODS: Using a large electronic health record database, we evaluated antiemetic prophylaxis as recommended in the guidelines and post-chemotherapy avoidable acute-care use (defined as involving any of NV or 8 other toxicities) for patients initiating carboplatin or other chemotherapy from October 2012 to August 2018. RESULTS: We identified 11,554 carboplatin courses. After the guideline change adding neurokinin-1 receptor antagonists (RAs) for carboplatin AUC ≥ 4, its use rose to 20% of courses from the prior average of 16%; virtually all courses also included a 5-HT3 RA plus dexamethasone. We found avoidable acute care in 23% of courses; one quarter of these events were associated with NV. Acute care rates after carboplatin mirrored those after other highly emetogenic chemotherapy or oxaliplatin and exceeded those after other chemotherapy regimens. The > 80% shortfall in adherence may have been caused by low awareness or acceptance of the guideline change and/or by poor awareness of avoidable acute-care use after carboplatin. CONCLUSION: Neurokinin-1 RA prophylaxis for carboplatin AUC ≥ 4 remains low and largely unchanged despite National Comprehensive Cancer Network and ASCO 2017 recommendations for inclusion. NV and avoidable acute care involving NV seen after carboplatin were consistent with other highly emetogenic chemotherapy. Clinician action is required to remediate incomplete prophylaxis and to no longer place patient outcomes, resources for cancer treatment, and clinician reimbursement at risk.