Leukemia is a type of blood cancer. It develops in the bone marrow and is the most common form of cancer in children. The condition affects around 3,800 children each year in the U.S., which is about 30 percent of childhood cancers. Leukemia can occur at any age, although it is most common in children aged 2 to 6 years old and found slightly more frequently in boys than girls. It is more common in Caucasian children than in children of other races.