The single normal kidney will grow faster and get larger than a normally paired kidney. For this reason, the single kidney is larger and heavier than normal, and it is, therefore, more vulnerable to injury. It is important to be aware of the increased risk for injury with certain heavy contact sports, so that careful decisions may be made regarding participation in various physical activities.In general, most people with a single normal kidney have few or no problems, particularly in the first few years. However, some longer term problems have been recognized. Most doctors believe that people with a single kidney, particularly from birth or during early childhood should be followed more closely than people with two normal kidneys. Children who have had a kidney surgically removed may have a slightly increased chance of developing abnormal amounts of protein in the urine.
Source : http://www.house.gov/mcdermott/kidneycaucus/onekidney.html