Background: to develop and validate a clinical decision rule (CDR) to predict one-month serious outcome and all-cause death in patients presenting with syncope to a UK Emergency Department (ED). Methods: This was a single centre prospective observational derivation and validation cohort design study of patients aged 16 years or over presenting to a UK ED with syncope. A CDR was devised using a derivation cohort and then tested on a validation cohort. Main Findings: Between 1^st March and 27^th October 2007, 550 patients were recruited into a derivation cohort. One-month serious outcome or all-cause death occurred in 40 (7.3%) patients and independent predictors were brain-type natriuretic peptide concentration ≥ 300 pg/mL, per rectum examination showing faecal occult blood, haemoglobin ≤ 90 g/L, oxygen saturation ≤ 94% on room air, Q wave except in lead III on presenting electrocardiogram, associated chest pain or bradycardia ≤ 50 /min at presentation. Between 27^th October 2007 and 22^nd July 2008, 550 patients were recruited into a validation cohort. One-month serious outcome or all-cause death occurred in 39 (7.1%) patients. The ROSE rule had a sensitivity and specificity of 87.2% and 65.5%, a negative predictive value of 98.5%. Other Findings: The ED management of syncope in the UK and Republic of Ireland is varied with only 18% of EDs having specific management guidelines. Troponin I predicts one-month serious outcome or all-cause death however acute myocardial infarction infrequently presents as syncope (0.7%) and can be diagnosed on presenting electrocardiogram. Plasma D-dimer is commonly raised and consequently does not predict one-month serious outcome or death. Interpretation: The ROSE rule has excellent negative predictive value and may be a valuable rule to help risk stratify patients presenting to the Emergency Department with syncope.