BACKGROUND: Emergency department (ED) ward admissions subsequently transferred to the intensive care unit (ICU) within 24 hours have higher mortality than direct ICU admissions. DESIGN, SETTING, PATIENTS: Describe risk factors for unplanned ICU transfer within 24 hours of ward arrival from the ED. METHODS: Evaluation of 178,315 ED non-ICU admissions to 13 US community hospitals. We tabulated the outcome of unplanned ICU transfer by patient characteristics and hospital volume. We present factors associated with unplanned ICU transfer after adjusting for patient and hospital differences in a hierarchical logistic regression. RESULTS: There were 4,252 (2.4%) non-ICU admissions transferred to the ICU within 24 hours. Admitting diagnoses most associated with unplanned transfer, listed by descending prevalence were: pneumonia (odds ratio [OR] 1.5; 95% confidence interval [CI] 1.2-1.9), myocardial infarction (MI) (OR 1.5; 95% CI 1.2-2.0), chronic obstructive pulmonary disease (COPD) (OR 1.4; 95% CI 1.1-1.9), sepsis (OR 2.5; 95% CI 1.9-3.3), and catastrophic conditions (OR 2.3; 95% CI 1.7-3.0). Other significant predictors included: male sex, Comorbidity Points Score >145, Laboratory Acute Physiology Score >/=7, arriving on the ward between 11 PM and 7 AM. Decreased risk was found with admission to monitored transitional care units (OR 0.83; 95% CI 0.77-0.90) and to higher volume hospitals (OR 0.94 per 1,000 additional annual ED inpatient admissions; 95% CI 0.91-0.98). CONCLUSIONS: ED patients admitted with respiratory conditions, MI, or sepsis are at modestly increased risk for unplanned ICU transfer and may benefit from better triage from the ED, earlier intervention, or closer monitoring to prevent acute decompensation. More research is needed to determine how intermediate care units, hospital volume, time of day, and sex affect unplanned ICU transfer. Journal of Hospital Medicine 2013. (c) 2012 Society of Hospital Medicine.