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Objective: To evaluate the prevalence and characteristics of acute kidney injury (AKI) in 100-km ultramarathon runners. Design: Prospective observational study. Setting: The 2011 Soochow University ultramarathon, in which each athlete ran for 100 km. Participants: All Taiwanese entrants who participated in the 100-km race and lived in the northern part of Taiwan were invited to participate in the study. Main Outcome Measures: Acute kidney injury was defined using the Acute Kidney Injury Network criteria. Blood and urine samples were collected 1 week before, immediately after, and 1 day after the race. Results: Immediately after the race, 85% (22) of the 26 subjects were diagnosed with AKI, 65% (16) with moderate dehydration, 23% (6) with muscle cramps, and 12% (3) with hematuria. Body weight was significantly decreased from prerace to all postrace measurements. Plasma levels of potassium ion, creatinine, renin, and aldosterone were significantly elevated immediately after the race and then significantly reduced 1 day after the race. Changes in plasma levels of sodium, creatine kinase, and creatine kinase–MB, as well as urine potassium and creatinine, were indicative of AKI. Conclusions: Transient AKI and muscle cramps are very common in 100-km ultramarathon runners. All transient ultra-runners who developed AKI in this study recovered their renal function 1 day later. Clinical Relevance: Ultramarathon running is associated with a wide range of significant changes in hematological parameters, several of which can be associated with potentially serious renal and physiological abnormalities. [ABSTRACT FROM AUTHOR]