Acute lymphoblastic leukemia (ALL) / lymphoblastic lymphoma (LBL), also known as acute lymphocytic leukemia / lymphoma or acute lymphoid leukemia, is a cancer of precursor B-cell, T-cell, or other cell types in which immature lymphoid cells accumulate in blood, bone marrow, or other tissue.

Diagnosis usually requires the presence of over 20% lymphoblasts in the peripheral blood and/or the presence of bone marrow or tissue infiltrate of immature cells with confirmation of lymphoid lineage by flow cytometry and/or cytochemistry.

ALL / LBL is the most common cancer in children (mostly younger than 6 years and accounting for a quarter of all childhood malignancies) and is also seen in adults (usually older than 60 years). Children may present with unexplained fevers, pallor, hepatosplenomegaly, lymphadenopathy, and/or bleeding and easy bruising. Retinal hemorrhage may occur. The cause of ALL / LBL is unknown, but it has been associated with ionizing radiation as well as certain genetic abnormalities. Approximately 75% of infants with ALL have KMT2A gene rearrangement (formerly known as mixed-lineage leukemia), associated with particularly poor outcomes. The Philadelphia chromosome (Ph), while rare in childhood cases of ALL, is present in 20%-25% of adult cases, the most common genetic subgroup. The incidence of Ph-positive ALL increases with age.