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Minimal requirements and their rationale for the diagnosis and the response to treatment in childhood acute lymphoblastic leukemia (ALL) were defined in the recently instituted "BFM-Family"-Group, in which the German, Austrian, Dutch, Italian, Belgian, French and Hungarian childhood leukemia study groups cooperate. ALL is defined as greater-than-or-equal-to 25% lymphoblasts in the bone marrow; for confirmation of the diagnosis and classification the criteria of the French-American-British (FAB) criteria are retained. For determination of the extent of the disease at diagnosis or relapse the criteria by the Rome Workshop [1986] are recommended: An obligatory panel of monoclonal antibodies for immunophenotyping was defined, as well as criteria for precursor B-ALL and T-ALL. Cytogenetic studies may support the diagnosis and subtyping, and are essential to identify certain patients with a high risk of treatment failure (f.i. t(9;22), t(4;11)). The role of molecular genetics for the diagnosis and the characterization of leukemia and the value of its clinical application needs further elucidation. Relapse was defined as recurrence of evident leukemia in the blood, bone marrow (greater-than-or-equal-to 25% lymphoblasts) or at any other site (to be confirmed by histological examination). Bone marrow involvement combined with extramedullary relapse was defined as greater-than-or-equal-to 5% lymphoblasts in the bone marrow.