Methods for diagnosing renal disorder, hemolytic uremic syndrome and enterohemorrhagic E. coli infectious disease at an early stage, and for diagnosing the severity of these diseases are provided. Detection of excretion of AQP2 and/or molecules derived therefrom into urine of a subject, or determination of the amount of excretion of the same is conducted to diagnose the morbidity or the severity of renal disorder, hemolytic uremic syndrome and enterohemorrhagic E. coli infectious disease of the subject. For the detection and determination of AQP2, an immunochemical process may be suitably employed in which an antibody that specifically recognizes AQP2.