Emergency department (ED) clinicians routinely decide the disposition of patients with suicidal ideation, with potential consequences for patient safety, liability, and system costs and resources. An expert consensus panel recently created a 6-item decision support tool for patients with passive or active suicidal ideation. Individuals scoring a 0 (exhibiting none of the tool’s 6 items) are considered “lower risk” and suitable for discharge, while those with non-0 scores are considered “elevated risk” and should receive further evaluation. The current study tested the predictive utility of this tool using existing data from the Emergency Department Safety Assessment and Follow-up Evaluation. ED patients with active suicide ideation (n = 1368) were followed for 12 months after an index visit using telephone assessment and medical chart review. About 1 in 5 patients had attempted suicide during follow-up. Because of the frequency of serious warning signs and risk factors in this population, only three patients met tool criteria for “lower risk” at baseline. The tool had perfect sensitivity, but exceptionally low specificity, in predicting suicidal behavior within 6 weeks and 12 months. In logistic regression analyses, several tool items were significantly associated with suicidal behavior within 6 weeks (suicide plan, past attempt) and 12 months (suicide plan, past attempt, suicide intent, significant mental health condition, irritability/agitation/aggression). Although the tool did not perform well as a binary instrument among those with active suicidal ideation, having a suicide plan identified almost all attempters while suicide plan and past attempt identified over four-fifths of near-term attempts.