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Background: Episodic collapse in horses has equine welfare and human safety implications. There are, however, no published case series describing this syndrome., Objectives: To characterize the cause and outcomes for horses referred for investigation of episodic collapse., Animals: Twenty-five horses referred for investigation of single or multiple episodes of collapse., Methods: Retrospective study. Clinical records from the Dick Vet Equine Hospital, University of Edinburgh from November 1995 to July 2009 were searched using the following keywords: collapse, collapsing, fall, syncope. Collapse was defined as an incident in which the horse lost postural tone with or without progression to recumbency and with or without loss of consciousness. Long-term follow-up information was obtained by telephone conversation with the owner., Results: A final diagnosis was reached in 11 cases, namely cardiac arrhythmia (4), right-sided heart failure (1), hypoglycemia (2), generalized seizures (2), and sleep disorder (2). A presumptive diagnosis was reached in 8 cases, namely neurocardiogenic syncope (5), exercise-induced pulmonary hemorrhage (2), and generalized seizures (1). No diagnosis was reached in 6 cases despite comprehensive investigations. Three horses were euthanized at presentation. Treatment was attempted in 9 horses with 6 cases having successful outcome before discharge. Follow-up information was available for 14 of 19 horses discharged from the hospital. Only 1 of these horses was observed to collapse after discharge., Conclusions and Clinical Importance: Definitive diagnosis was more likely to be reached in cases with multiple episodes of collapse. Horses in which 1 episode of collapse occurred did not necessarily collapse again., (Copyright © 2010 by the American College of Veterinary Internal Medicine.)