Acute lymphocytic leukemia (ALL) is responsible for 80% of the acute leukemias of childhood, with the peak incidence occurring between ages 3 and 7. ALL also occurs in adults, where it comprises 20% of all adult leukemias.

In acute leukemia, the malignant cell loses its ability to mature and specialize (differentiate) its function. These cells multiply rapidly and replace the normal cells. Bone marrow failure occurs as malignant cells replace normal bone marrow elements. The person becomes susceptible to bleeding and infection because the normal blood cells are reduced in number.

Most cases seem to have no apparent cause. However, radiation, some toxins such as benzene, and some chemotherapy agents are thought to contribute to the induction of leukemia. Abnormalities in chromosomes may also play a role in the development of acute leukemia.

Risk factors for acute leukemia include Down syndrome, a sibling with leukemia, and exposure to radiation, chemicals, and drugs. The incidence is 6 out of 100,000 people.