Patients with active cancer account for a growing percentage of all emergency department (ED) visits and have a unique set of risks related to their disease and its treatments. Effective triage for this population is fundamental to facilitating their emergency care. We evaluated the validity of the Emergency Severity Index (ESI; version 4) triage tool to predict ED-relevant outcomes among adult patients with active cancer. We conducted a prespecified analysis of the observational cohort established by the National Cancer Institute-supported Comprehensive Oncologic Emergencies Research Network's multicenter (18 sites) study of ED visits by patients with active cancer (N=1075). We used a series of χ2 tests for independence to relate ESI scores with 1) disposition, 2) ED resource use, 3) hospital length of stay, and 4) 30-day mortality. Among the 1008 subjects included in this analysis, the ESI distribution skewed heavily toward high acuity (>95% of subjects had an ESI level of 1, 2, or 3). ESI was significantly associated with patient disposition and ED resource use (p values<0.05). No significant associations were observed between ESI and the non-ED based outcomes of hospital length of stay or 30-day mortality. ESI scores among ED patients with active cancer indicate higher acuity than the general ED population and are predictive of disposition and ED resource use. These findings show that the ESI is a valid triage tool for use in this population for outcomes directly relevant to ED care.