Leukemia is the most common children cancer. It represents 25 to 30% of all new diagnosed cancer cases in children younger than 15 years in the world. Many new treatments over the last 50 years have improved survival at 80%, but the survivors face of lifelong diseases. Causal mechanisms for the occurrence of leukemia in children are poorly known, but it is often referred connection with environmental risk factors. Better knowledge of the causal mechanisms for the occurrence of leukemia in children is important for the prevention or early recognition and treatment of this cancer. Information European cancer registries indicate that the incidence rate of childhood leukemia between 1970 and 1999 grew on average by 0.7% per annum in the last twenty years has been 1% per year, mostly in countries with higher economic status (SES). There are marked differences between eastern and western European countries. In Slovenia, due to the small number of cases can not be described significant trend over the period 1998-2011.